diff --git a/.env.example b/.env.example
index 0fd9afe..376249d 100644
--- a/.env.example
+++ b/.env.example
@@ -51,6 +51,7 @@ BOOKING_BACK_SEAT_ENABLED=
 BOOKING_WHOLE_VEHICLE_ENABLED=
 BOOKING_FRONT_SEAT_MAX_PER_BOOKING=
 
+KBZ_APP_ID=
 KBZ_MERCHANT_CODE=
 KBZ_MERCHANT_KEY=
 KBZ_BASE_URL=
diff --git a/app-modules/booking/src/Models/Booking.php b/app-modules/booking/src/Models/Booking.php
index aaaca06..771b15f 100644
--- a/app-modules/booking/src/Models/Booking.php
+++ b/app-modules/booking/src/Models/Booking.php
@@ -11,6 +11,7 @@ use Modules\Booking\Database\Factories\BookingFactory;
 use Modules\Booking\Enums\BookingChannel;
 use Modules\Booking\Enums\BookingStatus;
 use Modules\Catalog\Models\DepartureTimeSlot;
+use Modules\Payment\Models\Payment;
 use Modules\Routing\Models\EvRoute;
 
 class Booking extends Model
@@ -85,4 +86,9 @@ class Booking extends Model
     {
         return $this->hasMany(BookingVehicleOption::class);
     }
+
+    public function payments(): HasMany
+    {
+        return $this->hasMany(Payment::class);
+    }
 }
diff --git a/app-modules/payment/database/factories/PaymentFactory.php b/app-modules/payment/database/factories/PaymentFactory.php
new file mode 100644
index 0000000..c838397
--- /dev/null
+++ b/app-modules/payment/database/factories/PaymentFactory.php
@@ -0,0 +1,54 @@
+<?php
+
+namespace Modules\Payment\Database\Factories;
+
+use Illuminate\Database\Eloquent\Factories\Factory;
+use Illuminate\Support\Str;
+use Modules\Booking\Models\Booking;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Models\Payment;
+
+/**
+ * @extends Factory<Payment>
+ */
+class PaymentFactory extends Factory
+{
+    protected $model = Payment::class;
+
+    /**
+     * Define the model's default state.
+     *
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        return [
+            'booking_id' => Booking::factory(),
+            'gateway' => PaymentMethod::KbzMiniApp,
+            'status' => PaymentStatus::Pending,
+            'amount' => $this->faker->randomFloat(2, 5000, 50000),
+            'currency' => 'MMK',
+            'gateway_transaction_id' => Str::uuid()->toString(),
+            'gateway_payload' => null,
+            'initiated_at' => now(),
+            'completed_at' => null,
+        ];
+    }
+
+    public function completed(): static
+    {
+        return $this->state(fn (array $attributes): array => [
+            'status' => PaymentStatus::Completed,
+            'completed_at' => now(),
+        ]);
+    }
+
+    public function failed(): static
+    {
+        return $this->state(fn (array $attributes): array => [
+            'status' => PaymentStatus::Failed,
+            'completed_at' => now(),
+        ]);
+    }
+}
diff --git a/app-modules/payment/database/factories/RefundFactory.php b/app-modules/payment/database/factories/RefundFactory.php
new file mode 100644
index 0000000..4d61726
--- /dev/null
+++ b/app-modules/payment/database/factories/RefundFactory.php
@@ -0,0 +1,52 @@
+<?php
+
+namespace Modules\Payment\Database\Factories;
+
+use Illuminate\Database\Eloquent\Factories\Factory;
+use Modules\Payment\Enums\RefundStatus;
+use Modules\Payment\Models\Payment;
+use Modules\Payment\Models\Refund;
+
+/**
+ * @extends Factory<Refund>
+ */
+class RefundFactory extends Factory
+{
+    protected $model = Refund::class;
+
+    /**
+     * Define the model's default state.
+     *
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        return [
+            'payment_id' => Payment::factory()->completed(),
+            'status' => RefundStatus::Pending,
+            'amount' => $this->faker->randomFloat(2, 1000, 50000),
+            'reason' => $this->faker->sentence(),
+            'gateway_refund_id' => null,
+            'gateway_payload' => null,
+            'requested_by' => null,
+            'requested_at' => now(),
+            'completed_at' => null,
+        ];
+    }
+
+    public function completed(): static
+    {
+        return $this->state(fn (array $attributes): array => [
+            'status' => RefundStatus::Completed,
+            'completed_at' => now(),
+        ]);
+    }
+
+    public function failed(): static
+    {
+        return $this->state(fn (array $attributes): array => [
+            'status' => RefundStatus::Failed,
+            'completed_at' => now(),
+        ]);
+    }
+}
diff --git a/app-modules/payment/database/migrations/2026_08_08_100000_create_payments_table.php b/app-modules/payment/database/migrations/2026_08_08_100000_create_payments_table.php
new file mode 100644
index 0000000..8726f92
--- /dev/null
+++ b/app-modules/payment/database/migrations/2026_08_08_100000_create_payments_table.php
@@ -0,0 +1,40 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * One row per payment attempt against a Booking — a Booking can have
+     * more than one row here if an earlier attempt failed and the customer
+     * retried (domain.md §1, §6).
+     */
+    public function up(): void
+    {
+        Schema::create('payments', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('booking_id')->constrained('bookings')->cascadeOnDelete();
+            $table->string('gateway');
+            $table->string('status')->default('pending');
+            $table->decimal('amount', 10, 2);
+            $table->string('currency')->default('MMK');
+            $table->string('gateway_transaction_id')->nullable()->index();
+            $table->jsonb('gateway_payload')->nullable();
+            $table->timestamp('initiated_at')->nullable();
+            $table->timestamp('completed_at')->nullable();
+            $table->timestamps();
+
+            $table->index(['booking_id', 'status']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('payments');
+    }
+};
diff --git a/app-modules/payment/database/migrations/2026_08_08_100100_create_refunds_table.php b/app-modules/payment/database/migrations/2026_08_08_100100_create_refunds_table.php
new file mode 100644
index 0000000..7eca79b
--- /dev/null
+++ b/app-modules/payment/database/migrations/2026_08_08_100100_create_refunds_table.php
@@ -0,0 +1,40 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * A Refund reverses a specific successful Payment, not the Booking
+     * directly — a Payment can have more than one Refund row for partial
+     * refunds (domain.md §1, §6).
+     */
+    public function up(): void
+    {
+        Schema::create('refunds', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('payment_id')->constrained('payments')->cascadeOnDelete();
+            $table->string('status')->default('pending');
+            $table->decimal('amount', 10, 2);
+            $table->text('reason');
+            $table->string('gateway_refund_id')->nullable()->index();
+            $table->jsonb('gateway_payload')->nullable();
+            $table->foreignId('requested_by')->nullable()->constrained('users')->nullOnDelete();
+            $table->timestamp('requested_at')->nullable();
+            $table->timestamp('completed_at')->nullable();
+            $table->timestamps();
+
+            $table->index(['payment_id', 'status']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('refunds');
+    }
+};
diff --git a/app-modules/payment/src/Contracts/PaymentGatewayInterface.php b/app-modules/payment/src/Contracts/PaymentGatewayInterface.php
new file mode 100644
index 0000000..4679211
--- /dev/null
+++ b/app-modules/payment/src/Contracts/PaymentGatewayInterface.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace Modules\Payment\Contracts;
+
+use Modules\Payment\Data\PaymentRequestData;
+use Modules\Payment\Data\PaymentResultData;
+use Modules\Payment\Data\RefundResultData;
+
+/**
+ * Contract every payment gateway strategy implements (e.g. KbzMiniAppGateway).
+ *
+ * Strategies are deliberately ignorant of Payment/Booking Eloquent models —
+ * they only take/return DTOs, so booking-status changes stay in listeners
+ * reacting to PaymentCompleted/PaymentFailed/RefundProcessed (domain.md §6).
+ */
+interface PaymentGatewayInterface
+{
+    /**
+     * Start a payment attempt with the gateway.
+     */
+    public function initiate(PaymentRequestData $data): PaymentResultData;
+
+    /**
+     * Re-check a payment's current status with the gateway (defense-in-depth
+     * re-verification, and used inside webhook processing — domain.md §6).
+     */
+    public function verify(string $gatewayTransactionId): PaymentResultData;
+
+    /**
+     * Reverse (all or part of) a successful payment.
+     */
+    public function refund(string $gatewayTransactionId, string $amount, string $reason): RefundResultData;
+}
diff --git a/app-modules/payment/src/Data/PaymentRequestData.php b/app-modules/payment/src/Data/PaymentRequestData.php
new file mode 100644
index 0000000..4a349ea
--- /dev/null
+++ b/app-modules/payment/src/Data/PaymentRequestData.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace Modules\Payment\Data;
+
+use Modules\Payment\Enums\PaymentMethod;
+
+/**
+ * What a gateway needs to start a payment (PaymentGatewayInterface::initiate).
+ */
+readonly class PaymentRequestData
+{
+    public function __construct(
+        public int $bookingId,
+        public string $merchantOrderId,
+        public string $amount,
+        public string $currency,
+        public PaymentMethod $method,
+        public ?string $notifyUrl = null,
+    ) {}
+}
diff --git a/app-modules/payment/src/Data/PaymentResultData.php b/app-modules/payment/src/Data/PaymentResultData.php
new file mode 100644
index 0000000..9f71533
--- /dev/null
+++ b/app-modules/payment/src/Data/PaymentResultData.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace Modules\Payment\Data;
+
+use Modules\Payment\Enums\PaymentStatus;
+
+/**
+ * What a gateway hands back from initiate()/verify() (PaymentGatewayInterface).
+ */
+readonly class PaymentResultData
+{
+    /**
+     * @param  array<string, mixed>  $gatewayPayload  Raw gateway response, persisted verbatim for audit.
+     */
+    public function __construct(
+        public PaymentStatus $status,
+        public ?string $gatewayTransactionId,
+        public array $gatewayPayload,
+        public ?string $message = null,
+    ) {}
+}
diff --git a/app-modules/payment/src/Data/RefundResultData.php b/app-modules/payment/src/Data/RefundResultData.php
new file mode 100644
index 0000000..21675ee
--- /dev/null
+++ b/app-modules/payment/src/Data/RefundResultData.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace Modules\Payment\Data;
+
+use Modules\Payment\Enums\RefundStatus;
+
+/**
+ * What a gateway hands back from refund() (PaymentGatewayInterface).
+ */
+readonly class RefundResultData
+{
+    /**
+     * @param  array<string, mixed>  $gatewayPayload  Raw gateway response, persisted verbatim for audit.
+     */
+    public function __construct(
+        public RefundStatus $status,
+        public ?string $gatewayRefundId,
+        public array $gatewayPayload,
+        public ?string $message = null,
+    ) {}
+}
diff --git a/app-modules/payment/src/Enums/PaymentMethod.php b/app-modules/payment/src/Enums/PaymentMethod.php
new file mode 100644
index 0000000..0165086
--- /dev/null
+++ b/app-modules/payment/src/Enums/PaymentMethod.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace Modules\Payment\Enums;
+
+/**
+ * Which gateway a Payment is processed through.
+ *
+ * Only KBZ Mini App is supported in v1 — the enum exists so
+ * PaymentGatewayFactory can resolve additional gateways later
+ * without call-site changes (domain.md §6, §7).
+ */
+enum PaymentMethod: string
+{
+    case KbzMiniApp = 'kbz_mini_app';
+}
diff --git a/app-modules/payment/src/Enums/PaymentStatus.php b/app-modules/payment/src/Enums/PaymentStatus.php
new file mode 100644
index 0000000..cb5d810
--- /dev/null
+++ b/app-modules/payment/src/Enums/PaymentStatus.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace Modules\Payment\Enums;
+
+enum PaymentStatus: string
+{
+    case Pending = 'pending';
+    case Completed = 'completed';
+    case Failed = 'failed';
+}
diff --git a/app-modules/payment/src/Enums/RefundStatus.php b/app-modules/payment/src/Enums/RefundStatus.php
new file mode 100644
index 0000000..d593f27
--- /dev/null
+++ b/app-modules/payment/src/Enums/RefundStatus.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace Modules\Payment\Enums;
+
+enum RefundStatus: string
+{
+    case Pending = 'pending';
+    case Completed = 'completed';
+    case Failed = 'failed';
+}
diff --git a/app-modules/payment/src/Factories/PaymentGatewayFactory.php b/app-modules/payment/src/Factories/PaymentGatewayFactory.php
new file mode 100644
index 0000000..09bfe5e
--- /dev/null
+++ b/app-modules/payment/src/Factories/PaymentGatewayFactory.php
@@ -0,0 +1,39 @@
+<?php
+
+namespace Modules\Payment\Factories;
+
+use Modules\Payment\Contracts\PaymentGatewayInterface;
+use Modules\Payment\Enums\PaymentMethod;
+use RuntimeException;
+
+/**
+ * Resolves a PaymentGatewayInterface implementation by PaymentMethod.
+ *
+ * Replaces bnf_event's 3x duplicated `switch($payment_type)` at each call
+ * site (domain.md §6). Call sites depend only on this factory, never on a
+ * concrete gateway class — swapping/adding a gateway is a `register()` call
+ * here, no controller/action changes.
+ */
+class PaymentGatewayFactory
+{
+    /**
+     * @var array<string, class-string<PaymentGatewayInterface>>
+     */
+    private array $bindings = [];
+
+    /**
+     * @param  class-string<PaymentGatewayInterface>  $gatewayClass
+     */
+    public function register(PaymentMethod $method, string $gatewayClass): void
+    {
+        $this->bindings[$method->value] = $gatewayClass;
+    }
+
+    public function make(PaymentMethod $method): PaymentGatewayInterface
+    {
+        $gatewayClass = $this->bindings[$method->value]
+            ?? throw new RuntimeException("No payment gateway registered for method [{$method->value}].");
+
+        return app($gatewayClass);
+    }
+}
diff --git a/app-modules/payment/src/Gateways/KbzMiniAppGateway.php b/app-modules/payment/src/Gateways/KbzMiniAppGateway.php
new file mode 100644
index 0000000..692d33a
--- /dev/null
+++ b/app-modules/payment/src/Gateways/KbzMiniAppGateway.php
@@ -0,0 +1,287 @@
+<?php
+
+namespace Modules\Payment\Gateways;
+
+use Illuminate\Http\Client\ConnectionException;
+use Illuminate\Support\Facades\Http;
+use Illuminate\Support\Str;
+use Modules\Payment\Contracts\PaymentGatewayInterface;
+use Modules\Payment\Data\PaymentRequestData;
+use Modules\Payment\Data\PaymentResultData;
+use Modules\Payment\Data\RefundResultData;
+use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Enums\RefundStatus;
+use Modules\Payment\Support\KbzSignature;
+
+/**
+ * KBZ Mini App gateway strategy, ported from bnf_event's
+ * `App\Strategies\Payments\KBZMiniApp`/`KBZPay` (domain.md §6).
+ *
+ * Reads credentials from config('services.kbz') by default; a config array
+ * may be injected directly (used by tests / the factory).
+ */
+class KbzMiniAppGateway implements PaymentGatewayInterface
+{
+    private readonly string $appId;
+
+    private readonly string $merchantCode;
+
+    private readonly string $merchantKey;
+
+    private readonly string $baseUrl;
+
+    private readonly ?string $notifyUrl;
+
+    private readonly ?string $certPath;
+
+    private readonly ?string $certKeyPath;
+
+    private readonly ?string $caPath;
+
+    private readonly ?string $certPassword;
+
+    /**
+     * @param  array<string, mixed>|null  $config
+     */
+    public function __construct(?array $config = null)
+    {
+        $config ??= (array) config('services.kbz');
+
+        $this->appId = (string) ($config['app_id'] ?? '');
+        $this->merchantCode = (string) ($config['merchant_code'] ?? '');
+        $this->merchantKey = (string) ($config['merchant_key'] ?? '');
+        $this->baseUrl = (string) ($config['base_url'] ?? '');
+        $this->notifyUrl = $config['notify_url'] ?? null;
+        $this->certPath = $config['cert_path'] ?? null;
+        $this->certKeyPath = $config['cert_key_path'] ?? null;
+        $this->caPath = $config['ca_path'] ?? null;
+        $this->certPassword = $config['cert_password'] ?? null;
+    }
+
+    public function initiate(PaymentRequestData $data): PaymentResultData
+    {
+        $params = $this->buildPrecreateParams($data);
+
+        try {
+            $response = Http::asJson()->post($this->baseUrl, ['Request' => $params]);
+        } catch (ConnectionException $exception) {
+            return new PaymentResultData(
+                status: PaymentStatus::Failed,
+                gatewayTransactionId: null,
+                gatewayPayload: [],
+                message: $exception->getMessage(),
+            );
+        }
+
+        /** @var array<string, mixed> $body */
+        $body = $response->json('Response', []);
+
+        if (! $response->successful() || ($body['result'] ?? null) !== 'SUCCESS') {
+            return new PaymentResultData(
+                status: PaymentStatus::Failed,
+                gatewayTransactionId: $body['prepay_id'] ?? null,
+                gatewayPayload: $body,
+                message: $body['msg'] ?? 'KBZ precreate failed.',
+            );
+        }
+
+        return new PaymentResultData(
+            // KBZ's queryorder/refund calls both key off our own merch_order_id,
+            // not their prepay_id — so that's what gets stored/passed forward as
+            // the gateway transaction id (prepay_id still lives in the payload).
+            status: PaymentStatus::Pending,
+            gatewayTransactionId: $data->merchantOrderId,
+            gatewayPayload: $body,
+        );
+    }
+
+    public function verify(string $gatewayTransactionId): PaymentResultData
+    {
+        $params = $this->buildQueryOrderParams($gatewayTransactionId);
+
+        try {
+            $response = Http::asJson()->post($this->baseUrl, ['Request' => $params]);
+        } catch (ConnectionException $exception) {
+            return new PaymentResultData(
+                status: PaymentStatus::Failed,
+                gatewayTransactionId: $gatewayTransactionId,
+                gatewayPayload: [],
+                message: $exception->getMessage(),
+            );
+        }
+
+        /** @var array<string, mixed> $body */
+        $body = $response->json('Response', []);
+
+        return new PaymentResultData(
+            status: $this->mapTradeStatus($body['trade_status'] ?? null),
+            gatewayTransactionId: $gatewayTransactionId,
+            gatewayPayload: $body,
+            message: $body['trade_status'] ?? null,
+        );
+    }
+
+    public function refund(string $gatewayTransactionId, string $amount, string $reason): RefundResultData
+    {
+        $params = $this->buildRefundParams($gatewayTransactionId, $amount, $reason);
+
+        try {
+            $response = Http::asJson()
+                ->withOptions($this->mtlsOptions())
+                ->post($this->baseUrl, ['Request' => $params]);
+        } catch (ConnectionException $exception) {
+            return new RefundResultData(
+                status: RefundStatus::Failed,
+                gatewayRefundId: null,
+                gatewayPayload: [],
+                message: $exception->getMessage(),
+            );
+        }
+
+        /** @var array<string, mixed> $body */
+        $body = $response->json('Response', []);
+
+        if (! $response->successful() || ($body['result'] ?? null) !== 'SUCCESS') {
+            return new RefundResultData(
+                status: RefundStatus::Failed,
+                gatewayRefundId: $body['refund_order_id'] ?? null,
+                gatewayPayload: $body,
+                message: $body['msg'] ?? 'KBZ refund failed.',
+            );
+        }
+
+        return new RefundResultData(
+            status: RefundStatus::Completed,
+            gatewayRefundId: $body['refund_order_id'] ?? null,
+            gatewayPayload: $body,
+        );
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    private function buildPrecreateParams(PaymentRequestData $data): array
+    {
+        $params = [
+            'timestamp' => (string) now()->timestamp,
+            'method' => 'kbz.payment.precreate',
+            'notify_url' => $data->notifyUrl ?? $this->notifyUrl,
+            'nonce_str' => (string) Str::uuid(),
+            'version' => '1.0',
+            'biz_content' => [
+                'appid' => $this->appId,
+                'merch_code' => $this->merchantCode,
+                'merch_order_id' => $data->merchantOrderId,
+                'trade_type' => 'MINIAPP',
+                'total_amount' => $data->amount,
+                'trans_currency' => $data->currency,
+                'callback_info' => 'urlencode',
+            ],
+        ];
+
+        $params['sign'] = KbzSignature::sign($params, $this->merchantKey);
+        $params['sign_type'] = 'SHA256';
+
+        return $params;
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    private function buildQueryOrderParams(string $merchantOrderId): array
+    {
+        $params = [
+            'timestamp' => (string) now()->timestamp,
+            'method' => 'kbz.payment.queryorder',
+            'nonce_str' => (string) Str::uuid(),
+            'version' => '1.0',
+            'biz_content' => [
+                'appid' => $this->appId,
+                'merch_code' => $this->merchantCode,
+                'merch_order_id' => $merchantOrderId,
+            ],
+        ];
+
+        $params['sign'] = KbzSignature::sign($params, $this->merchantKey);
+        $params['sign_type'] = 'SHA256';
+
+        return $params;
+    }
+
+    /**
+     * KBZ's queryorder trade_status values — mapped conservatively: anything
+     * not explicitly a success/pending state is treated as failed rather
+     * than silently left as an unhandled status (domain.md §6).
+     */
+    private function mapTradeStatus(?string $tradeStatus): PaymentStatus
+    {
+        return match ($tradeStatus) {
+            'PAY_SUCCESS' => PaymentStatus::Completed,
+            'WAIT_PAY', 'USERPAYING' => PaymentStatus::Pending,
+            default => PaymentStatus::Failed,
+        };
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    private function buildRefundParams(string $merchantOrderId, string $amount, string $reason): array
+    {
+        $params = [
+            'timestamp' => (string) now()->timestamp,
+            'method' => 'kbz.payment.refund',
+            'nonce_str' => (string) Str::uuid(),
+            'version' => '1.0',
+            'biz_content' => [
+                'appid' => $this->appId,
+                'merch_code' => $this->merchantCode,
+                'merch_order_id' => $merchantOrderId,
+                'refund_request_no' => $this->refundRequestNo(),
+                // Unlike bnf_event (refund_amount was commented out, full-refund
+                // only), this is wired through to support partial refunds —
+                // domain.md §6.
+                'refund_amount' => $amount,
+                'refund_reason' => $reason,
+            ],
+        ];
+
+        $params['sign'] = KbzSignature::sign($params, $this->merchantKey);
+        $params['sign_type'] = 'SHA256';
+
+        return $params;
+    }
+
+    private function refundRequestNo(): string
+    {
+        return now()->format('YmdHi').strtoupper(Str::random(8));
+    }
+
+    /**
+     * mTLS options for the refund call — KBZ requires a client cert/key +
+     * CA bundle on `kbz.payment.refund` specifically (domain.md §6).
+     *
+     * @return array<string, mixed>
+     */
+    private function mtlsOptions(): array
+    {
+        $options = [];
+
+        if ($this->certPath !== null) {
+            $options['cert'] = $this->certPassword !== null
+                ? [$this->certPath, $this->certPassword]
+                : $this->certPath;
+        }
+
+        if ($this->certKeyPath !== null) {
+            $options['ssl_key'] = $this->certPassword !== null
+                ? [$this->certKeyPath, $this->certPassword]
+                : $this->certKeyPath;
+        }
+
+        if ($this->caPath !== null) {
+            $options['verify'] = $this->caPath;
+        }
+
+        return $options;
+    }
+}
diff --git a/app-modules/payment/src/Models/Payment.php b/app-modules/payment/src/Models/Payment.php
new file mode 100644
index 0000000..b40c557
--- /dev/null
+++ b/app-modules/payment/src/Models/Payment.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace Modules\Payment\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+use Modules\Booking\Models\Booking;
+use Modules\Payment\Database\Factories\PaymentFactory;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Enums\PaymentStatus;
+
+/**
+ * One attempt to pay for a Booking through a gateway — a Booking can have
+ * more than one Payment row if an earlier attempt failed and the customer
+ * retried (domain.md §1).
+ */
+class Payment extends Model
+{
+    /** @use HasFactory<PaymentFactory> */
+    use HasFactory;
+
+    /**
+     * @var list<string>
+     */
+    protected $fillable = [
+        'booking_id',
+        'gateway',
+        'status',
+        'amount',
+        'currency',
+        'gateway_transaction_id',
+        'gateway_payload',
+        'initiated_at',
+        'completed_at',
+    ];
+
+    /**
+     * @return array<string, string>
+     */
+    protected function casts(): array
+    {
+        return [
+            'gateway' => PaymentMethod::class,
+            'status' => PaymentStatus::class,
+            'amount' => 'decimal:2',
+            'gateway_payload' => 'array',
+            'initiated_at' => 'datetime',
+            'completed_at' => 'datetime',
+        ];
+    }
+
+    public function booking(): BelongsTo
+    {
+        return $this->belongsTo(Booking::class);
+    }
+
+    public function refunds(): HasMany
+    {
+        return $this->hasMany(Refund::class);
+    }
+}
diff --git a/app-modules/payment/src/Models/Refund.php b/app-modules/payment/src/Models/Refund.php
new file mode 100644
index 0000000..c8160ff
--- /dev/null
+++ b/app-modules/payment/src/Models/Refund.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Modules\Payment\Models;
+
+use App\Models\User;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Modules\Payment\Database\Factories\RefundFactory;
+use Modules\Payment\Enums\RefundStatus;
+
+/**
+ * A reversal against a specific successful Payment (not against the Booking
+ * directly) — a Payment can have more than one Refund row for partial
+ * refunds (domain.md §1, §6).
+ */
+class Refund extends Model
+{
+    /** @use HasFactory<RefundFactory> */
+    use HasFactory;
+
+    /**
+     * @var list<string>
+     */
+    protected $fillable = [
+        'payment_id',
+        'status',
+        'amount',
+        'reason',
+        'gateway_refund_id',
+        'gateway_payload',
+        'requested_by',
+        'requested_at',
+        'completed_at',
+    ];
+
+    /**
+     * @return array<string, string>
+     */
+    protected function casts(): array
+    {
+        return [
+            'status' => RefundStatus::class,
+            'amount' => 'decimal:2',
+            'gateway_payload' => 'array',
+            'requested_at' => 'datetime',
+            'completed_at' => 'datetime',
+        ];
+    }
+
+    public function payment(): BelongsTo
+    {
+        return $this->belongsTo(Payment::class);
+    }
+
+    public function requestedBy(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'requested_by');
+    }
+}
diff --git a/app-modules/payment/src/Providers/PaymentServiceProvider.php b/app-modules/payment/src/Providers/PaymentServiceProvider.php
index f2ae1c2..8a027c9 100644
--- a/app-modules/payment/src/Providers/PaymentServiceProvider.php
+++ b/app-modules/payment/src/Providers/PaymentServiceProvider.php
@@ -3,10 +3,21 @@
 namespace Modules\Payment\Providers;
 
 use Illuminate\Support\ServiceProvider;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Factories\PaymentGatewayFactory;
+use Modules\Payment\Gateways\KbzMiniAppGateway;
 
 class PaymentServiceProvider extends ServiceProvider
 {
-    public function register(): void {}
+    public function register(): void
+    {
+        $this->app->singleton(PaymentGatewayFactory::class, function (): PaymentGatewayFactory {
+            $factory = new PaymentGatewayFactory;
+            $factory->register(PaymentMethod::KbzMiniApp, KbzMiniAppGateway::class);
+
+            return $factory;
+        });
+    }
 
     public function boot(): void {}
 }
diff --git a/app-modules/payment/src/Services/PaymentService.php b/app-modules/payment/src/Services/PaymentService.php
new file mode 100644
index 0000000..75a5ac4
--- /dev/null
+++ b/app-modules/payment/src/Services/PaymentService.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace Modules\Payment\Services;
+
+use Modules\Payment\Data\PaymentRequestData;
+use Modules\Payment\Data\PaymentResultData;
+use Modules\Payment\Data\RefundResultData;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Factories\PaymentGatewayFactory;
+
+/**
+ * Thin orchestrator delegating to the gateway resolved by
+ * PaymentGatewayFactory — the single call site every Payment Action goes
+ * through, so no Action ever depends on a concrete gateway class.
+ */
+class PaymentService
+{
+    public function __construct(
+        private readonly PaymentGatewayFactory $gateways,
+    ) {}
+
+    public function initiate(PaymentRequestData $data): PaymentResultData
+    {
+        return $this->gateways->make($data->method)->initiate($data);
+    }
+
+    public function verify(PaymentMethod $method, string $gatewayTransactionId): PaymentResultData
+    {
+        return $this->gateways->make($method)->verify($gatewayTransactionId);
+    }
+
+    public function refund(PaymentMethod $method, string $gatewayTransactionId, string $amount, string $reason): RefundResultData
+    {
+        return $this->gateways->make($method)->refund($gatewayTransactionId, $amount, $reason);
+    }
+}
diff --git a/app-modules/payment/src/Support/KbzSignature.php b/app-modules/payment/src/Support/KbzSignature.php
new file mode 100644
index 0000000..6b1a4bf
--- /dev/null
+++ b/app-modules/payment/src/Support/KbzSignature.php
@@ -0,0 +1,72 @@
+<?php
+
+namespace Modules\Payment\Support;
+
+/**
+ * KBZ's signing scheme, ported verbatim from bnf_event's `KBZPay::joinKeyVal`/
+ * `signature` (domain.md §6): flatten the request array (excluding `sign`/
+ * `sign_type`, at any nesting level — `biz_content` included) into sorted
+ * `key=val` pairs joined by `&`, append `&key={merchant_key}`, SHA-256 hash,
+ * uppercase. Shared by initiate()/verify()/refund() on every gateway.
+ */
+class KbzSignature
+{
+    /**
+     * @param  array<string, mixed>  $data
+     * @param  list<string>  $skips  Additional top-level/nested keys to exclude beyond sign/sign_type.
+     */
+    public static function joinKeyVal(array $data, array $skips = []): string
+    {
+        $skips = [...$skips, 'sign', 'sign_type'];
+
+        $fields = [];
+        self::collect($data, $skips, $fields);
+
+        usort($fields, fn (array $a, array $b): int => strcmp($a['key'], $b['key']));
+
+        $pairs = [];
+        foreach ($fields as $field) {
+            if ($field['val'] !== null && trim((string) $field['val']) !== '') {
+                $pairs[] = $field['key'].'='.$field['val'];
+            }
+        }
+
+        return implode('&', $pairs);
+    }
+
+    /**
+     * @param  array<string, mixed>  $data
+     * @param  list<string>  $skips
+     */
+    public static function sign(array $data, string $merchantKey, array $skips = []): string
+    {
+        $joined = self::joinKeyVal($data, $skips);
+
+        return strtoupper(hash('sha256', $joined.'&key='.$merchantKey));
+    }
+
+    /**
+     * @param  list<string>  $skips
+     * @param  list<array{key: string, val: mixed}>  $fields
+     */
+    private static function collect(mixed $value, array $skips, array &$fields, string $key = ''): void
+    {
+        if (in_array($key, $skips, true)) {
+            return;
+        }
+
+        if (is_array($value)) {
+            foreach ($value as $subKey => $subVal) {
+                self::collect($subVal, $skips, $fields, (string) $subKey);
+            }
+
+            return;
+        }
+
+        if ($key === '') {
+            return;
+        }
+
+        $fields[] = ['key' => $key, 'val' => $value];
+    }
+}
diff --git a/app-modules/payment/tests/Feature/KbzMiniAppGatewayInitiateTest.php b/app-modules/payment/tests/Feature/KbzMiniAppGatewayInitiateTest.php
new file mode 100644
index 0000000..f68ec8d
--- /dev/null
+++ b/app-modules/payment/tests/Feature/KbzMiniAppGatewayInitiateTest.php
@@ -0,0 +1,77 @@
+<?php
+
+use Illuminate\Http\Client\ConnectionException;
+use Illuminate\Support\Facades\Http;
+use Modules\Payment\Data\PaymentRequestData;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Gateways\KbzMiniAppGateway;
+use Modules\Payment\Support\KbzSignature;
+
+$config = [
+    'app_id' => 'APPID123',
+    'merchant_code' => 'MERCH001',
+    'merchant_key' => 'test-merchant-key',
+    'base_url' => 'https://kbz.test/gateway',
+    'notify_url' => 'https://app.test/api/v1/webhooks/kbz',
+];
+
+$paymentRequest = new PaymentRequestData(
+    bookingId: 1,
+    merchantOrderId: 'EVB-FIXTURE-001',
+    amount: '15000',
+    currency: 'MMK',
+    method: PaymentMethod::KbzMiniApp,
+);
+
+test('initiate posts a correctly signed precreate request to the configured base_url', function () use ($config, $paymentRequest) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['result' => 'SUCCESS', 'prepay_id' => 'PREPAY123']])]);
+
+    (new KbzMiniAppGateway($config))->initiate($paymentRequest);
+
+    Http::assertSent(function ($request) {
+        $body = $request->data()['Request'];
+
+        return $request->url() === 'https://kbz.test/gateway'
+            && $body['method'] === 'kbz.payment.precreate'
+            && $body['sign_type'] === 'SHA256'
+            && $body['biz_content']['appid'] === 'APPID123'
+            && $body['biz_content']['merch_code'] === 'MERCH001'
+            && $body['biz_content']['merch_order_id'] === 'EVB-FIXTURE-001'
+            && $body['biz_content']['trade_type'] === 'MINIAPP'
+            && $body['biz_content']['total_amount'] === '15000'
+            && $body['biz_content']['trans_currency'] === 'MMK'
+            && $body['sign'] === KbzSignature::sign($body, 'test-merchant-key');
+    });
+});
+
+test('initiate returns a pending PaymentResultData on a successful precreate', function () use ($config, $paymentRequest) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['result' => 'SUCCESS', 'prepay_id' => 'PREPAY123']])]);
+
+    $result = (new KbzMiniAppGateway($config))->initiate($paymentRequest);
+
+    expect($result->status)->toBe(PaymentStatus::Pending)
+        ->and($result->gatewayTransactionId)->toBe('EVB-FIXTURE-001')
+        ->and($result->gatewayPayload)->toBe(['result' => 'SUCCESS', 'prepay_id' => 'PREPAY123']);
+});
+
+test('initiate returns a failed PaymentResultData when KBZ rejects the request', function () use ($config, $paymentRequest) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['result' => 'FAIL', 'msg' => 'Invalid merchant']])]);
+
+    $result = (new KbzMiniAppGateway($config))->initiate($paymentRequest);
+
+    expect($result->status)->toBe(PaymentStatus::Failed)
+        ->and($result->gatewayTransactionId)->toBeNull()
+        ->and($result->message)->toBe('Invalid merchant');
+});
+
+test('initiate returns a failed PaymentResultData when the connection fails', function () use ($config, $paymentRequest) {
+    Http::fake(['kbz.test/*' => fn () => throw new ConnectionException('Connection refused')]);
+
+    $result = (new KbzMiniAppGateway($config))->initiate($paymentRequest);
+
+    expect($result->status)->toBe(PaymentStatus::Failed)
+        ->and($result->gatewayTransactionId)->toBeNull()
+        ->and($result->gatewayPayload)->toBe([])
+        ->and($result->message)->toBe('Connection refused');
+});
diff --git a/app-modules/payment/tests/Feature/KbzMiniAppGatewayRefundTest.php b/app-modules/payment/tests/Feature/KbzMiniAppGatewayRefundTest.php
new file mode 100644
index 0000000..0bee9c6
--- /dev/null
+++ b/app-modules/payment/tests/Feature/KbzMiniAppGatewayRefundTest.php
@@ -0,0 +1,97 @@
+<?php
+
+use Illuminate\Http\Client\ConnectionException;
+use Illuminate\Support\Facades\Http;
+use Modules\Payment\Enums\RefundStatus;
+use Modules\Payment\Gateways\KbzMiniAppGateway;
+use Modules\Payment\Support\KbzSignature;
+
+$config = [
+    'app_id' => 'APPID123',
+    'merchant_code' => 'MERCH001',
+    'merchant_key' => 'test-merchant-key',
+    'base_url' => 'https://kbz.test/gateway',
+    'notify_url' => 'https://app.test/api/v1/webhooks/kbz',
+];
+
+test('refund posts a correctly signed refund request, wiring the partial amount through', function () use ($config) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['result' => 'SUCCESS', 'refund_order_id' => 'REFUND123']])]);
+
+    (new KbzMiniAppGateway($config))->refund('EVB-FIXTURE-001', '8000', 'customer requested partial refund');
+
+    Http::assertSent(function ($request) {
+        $body = $request->data()['Request'];
+
+        return $request->url() === 'https://kbz.test/gateway'
+            && $body['method'] === 'kbz.payment.refund'
+            && $body['sign_type'] === 'SHA256'
+            && $body['biz_content']['appid'] === 'APPID123'
+            && $body['biz_content']['merch_code'] === 'MERCH001'
+            && $body['biz_content']['merch_order_id'] === 'EVB-FIXTURE-001'
+            // unlike bnf_event (amount hardcoded/commented out), this is wired through
+            && $body['biz_content']['refund_amount'] === '8000'
+            && $body['biz_content']['refund_reason'] === 'customer requested partial refund'
+            && ! empty($body['biz_content']['refund_request_no'])
+            && $body['sign'] === KbzSignature::sign($body, 'test-merchant-key');
+    });
+});
+
+test('refund returns a completed RefundResultData on success', function () use ($config) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['result' => 'SUCCESS', 'refund_order_id' => 'REFUND123']])]);
+
+    $result = (new KbzMiniAppGateway($config))->refund('EVB-FIXTURE-001', '8000', 'customer request');
+
+    expect($result->status)->toBe(RefundStatus::Completed)
+        ->and($result->gatewayRefundId)->toBe('REFUND123')
+        ->and($result->gatewayPayload)->toBe(['result' => 'SUCCESS', 'refund_order_id' => 'REFUND123']);
+});
+
+test('refund returns a failed RefundResultData when KBZ rejects the request', function () use ($config) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['result' => 'FAIL', 'msg' => 'Refund window expired']])]);
+
+    $result = (new KbzMiniAppGateway($config))->refund('EVB-FIXTURE-001', '8000', 'customer request');
+
+    expect($result->status)->toBe(RefundStatus::Failed)
+        ->and($result->gatewayRefundId)->toBeNull()
+        ->and($result->message)->toBe('Refund window expired');
+});
+
+test('refund returns a failed RefundResultData when the connection fails', function () use ($config) {
+    Http::fake(['kbz.test/*' => fn () => throw new ConnectionException('Connection refused')]);
+
+    $result = (new KbzMiniAppGateway($config))->refund('EVB-FIXTURE-001', '8000', 'customer request');
+
+    expect($result->status)->toBe(RefundStatus::Failed)
+        ->and($result->gatewayRefundId)->toBeNull()
+        ->and($result->gatewayPayload)->toBe([])
+        ->and($result->message)->toBe('Connection refused');
+});
+
+test('refund builds mTLS cert/ssl_key/verify options from config', function () {
+    $gateway = new KbzMiniAppGateway([
+        'app_id' => 'APPID123',
+        'merchant_code' => 'MERCH001',
+        'merchant_key' => 'test-merchant-key',
+        'base_url' => 'https://kbz.test/gateway',
+        'cert_path' => '/certs/merch.pem',
+        'cert_key_path' => '/certs/merch.key',
+        'ca_path' => '/certs/ca.crt',
+        'cert_password' => 'secret',
+    ]);
+
+    $options = (new ReflectionMethod($gateway, 'mtlsOptions'))->invoke($gateway);
+
+    expect($options)->toBe([
+        'cert' => ['/certs/merch.pem', 'secret'],
+        'ssl_key' => ['/certs/merch.key', 'secret'],
+        'verify' => '/certs/ca.crt',
+    ]);
+});
+
+test('refund omits mTLS options entirely when cert config is not set', function () use ($config) {
+    $gateway = new KbzMiniAppGateway($config);
+
+    $options = (new ReflectionMethod($gateway, 'mtlsOptions'))->invoke($gateway);
+
+    expect($options)->toBe([]);
+});
diff --git a/app-modules/payment/tests/Feature/KbzMiniAppGatewayVerifyTest.php b/app-modules/payment/tests/Feature/KbzMiniAppGatewayVerifyTest.php
new file mode 100644
index 0000000..20f62e2
--- /dev/null
+++ b/app-modules/payment/tests/Feature/KbzMiniAppGatewayVerifyTest.php
@@ -0,0 +1,72 @@
+<?php
+
+use Illuminate\Http\Client\ConnectionException;
+use Illuminate\Support\Facades\Http;
+use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Gateways\KbzMiniAppGateway;
+use Modules\Payment\Support\KbzSignature;
+
+$config = [
+    'app_id' => 'APPID123',
+    'merchant_code' => 'MERCH001',
+    'merchant_key' => 'test-merchant-key',
+    'base_url' => 'https://kbz.test/gateway',
+    'notify_url' => 'https://app.test/api/v1/webhooks/kbz',
+];
+
+test('verify posts a correctly signed queryorder request keyed by our own merch_order_id', function () use ($config) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['trade_status' => 'PAY_SUCCESS']])]);
+
+    (new KbzMiniAppGateway($config))->verify('EVB-FIXTURE-001');
+
+    Http::assertSent(function ($request) {
+        $body = $request->data()['Request'];
+
+        return $request->url() === 'https://kbz.test/gateway'
+            && $body['method'] === 'kbz.payment.queryorder'
+            && $body['sign_type'] === 'SHA256'
+            && $body['biz_content']['appid'] === 'APPID123'
+            && $body['biz_content']['merch_code'] === 'MERCH001'
+            && $body['biz_content']['merch_order_id'] === 'EVB-FIXTURE-001'
+            && ! array_key_exists('total_amount', $body['biz_content'])
+            && $body['sign'] === KbzSignature::sign($body, 'test-merchant-key');
+    });
+});
+
+test('verify maps PAY_SUCCESS to a completed PaymentResultData', function () use ($config) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['trade_status' => 'PAY_SUCCESS', 'mm_order_id' => 'MM123']])]);
+
+    $result = (new KbzMiniAppGateway($config))->verify('EVB-FIXTURE-001');
+
+    expect($result->status)->toBe(PaymentStatus::Completed)
+        ->and($result->gatewayTransactionId)->toBe('EVB-FIXTURE-001')
+        ->and($result->gatewayPayload)->toBe(['trade_status' => 'PAY_SUCCESS', 'mm_order_id' => 'MM123']);
+});
+
+test('verify maps WAIT_PAY to a pending PaymentResultData', function () use ($config) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['trade_status' => 'WAIT_PAY']])]);
+
+    $result = (new KbzMiniAppGateway($config))->verify('EVB-FIXTURE-001');
+
+    expect($result->status)->toBe(PaymentStatus::Pending);
+});
+
+test('verify maps any other/unrecognized trade_status to a failed PaymentResultData', function () use ($config) {
+    Http::fake(['kbz.test/*' => Http::response(['Response' => ['trade_status' => 'PAY_ERROR']])]);
+
+    $result = (new KbzMiniAppGateway($config))->verify('EVB-FIXTURE-001');
+
+    expect($result->status)->toBe(PaymentStatus::Failed)
+        ->and($result->message)->toBe('PAY_ERROR');
+});
+
+test('verify returns a failed PaymentResultData when the connection fails', function () use ($config) {
+    Http::fake(['kbz.test/*' => fn () => throw new ConnectionException('Connection refused')]);
+
+    $result = (new KbzMiniAppGateway($config))->verify('EVB-FIXTURE-001');
+
+    expect($result->status)->toBe(PaymentStatus::Failed)
+        ->and($result->gatewayTransactionId)->toBe('EVB-FIXTURE-001')
+        ->and($result->gatewayPayload)->toBe([])
+        ->and($result->message)->toBe('Connection refused');
+});
diff --git a/app-modules/payment/tests/Feature/PaymentAndRefundTest.php b/app-modules/payment/tests/Feature/PaymentAndRefundTest.php
new file mode 100644
index 0000000..95131b8
--- /dev/null
+++ b/app-modules/payment/tests/Feature/PaymentAndRefundTest.php
@@ -0,0 +1,88 @@
+<?php
+
+use Illuminate\Database\QueryException;
+use Modules\Booking\Models\Booking;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Enums\RefundStatus;
+use Modules\Payment\Models\Payment;
+use Modules\Payment\Models\Refund;
+
+test('a payment belongs to a booking', function () {
+    $booking = Booking::factory()->create();
+
+    $payment = Payment::factory()->create(['booking_id' => $booking->id]);
+
+    expect($payment->booking)->toBeInstanceOf(Booking::class)
+        ->and($payment->booking->is($booking))->toBeTrue()
+        ->and($booking->payments->first()->is($payment))->toBeTrue();
+});
+
+test('a booking can have more than one payment attempt', function () {
+    $booking = Booking::factory()->create();
+
+    Payment::factory()->failed()->create(['booking_id' => $booking->id]);
+    Payment::factory()->completed()->create(['booking_id' => $booking->id]);
+
+    expect($booking->payments)->toHaveCount(2);
+});
+
+test('gateway and status cast to their enums', function () {
+    $payment = Payment::factory()->create([
+        'gateway' => PaymentMethod::KbzMiniApp,
+        'status' => PaymentStatus::Completed,
+    ]);
+
+    expect($payment->gateway)->toBe(PaymentMethod::KbzMiniApp)
+        ->and($payment->status)->toBe(PaymentStatus::Completed);
+});
+
+test('a payment defaults to pending', function () {
+    $payment = Payment::factory()->create();
+
+    expect($payment->status)->toBe(PaymentStatus::Pending);
+});
+
+test('a refund belongs to a payment, not the booking directly', function () {
+    $payment = Payment::factory()->completed()->create();
+
+    $refund = Refund::factory()->create(['payment_id' => $payment->id]);
+
+    expect($refund->payment)->toBeInstanceOf(Payment::class)
+        ->and($refund->payment->is($payment))->toBeTrue()
+        ->and($payment->refunds->first()->is($refund))->toBeTrue();
+});
+
+test('a payment can have more than one refund for partial refunds', function () {
+    $payment = Payment::factory()->completed()->create(['amount' => 20000]);
+
+    Refund::factory()->completed()->create(['payment_id' => $payment->id, 'amount' => 8000]);
+    Refund::factory()->create(['payment_id' => $payment->id, 'amount' => 12000]);
+
+    expect($payment->refunds)->toHaveCount(2);
+});
+
+test('refund status casts to its enum and defaults to pending', function () {
+    $refund = Refund::factory()->create();
+
+    expect($refund->status)->toBe(RefundStatus::Pending);
+});
+
+test('deleting a payment cascades to its refunds', function () {
+    $payment = Payment::factory()->completed()->create();
+    $refund = Refund::factory()->create(['payment_id' => $payment->id]);
+
+    $payment->delete();
+
+    expect(Refund::find($refund->id))->toBeNull();
+});
+
+test('gateway_transaction_id cannot be shared across unrelated retried attempts without being unique-constrained', function () {
+    // gateway_transaction_id is not unique-constrained since a failed attempt
+    // may legitimately be retried under a fresh Payment row with its own id;
+    // this just documents the column accepts duplicates without throwing.
+    Payment::factory()->create(['gateway_transaction_id' => 'kbz-txn-1']);
+
+    expect(fn () => Payment::factory()->create(['gateway_transaction_id' => 'kbz-txn-1']))
+        ->not->toThrow(QueryException::class);
+});
diff --git a/app-modules/payment/tests/Feature/PaymentGatewayFactoryTest.php b/app-modules/payment/tests/Feature/PaymentGatewayFactoryTest.php
new file mode 100644
index 0000000..a1b9994
--- /dev/null
+++ b/app-modules/payment/tests/Feature/PaymentGatewayFactoryTest.php
@@ -0,0 +1,60 @@
+<?php
+
+use Modules\Payment\Contracts\PaymentGatewayInterface;
+use Modules\Payment\Data\PaymentRequestData;
+use Modules\Payment\Data\PaymentResultData;
+use Modules\Payment\Data\RefundResultData;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Factories\PaymentGatewayFactory;
+use Modules\Payment\Gateways\KbzMiniAppGateway;
+
+/**
+ * Stands in for "a second gateway" being added — proves call sites that only
+ * depend on PaymentGatewayFactory (never a concrete gateway class) don't
+ * need to change when a gateway implementation is swapped/added.
+ */
+class FakePaymentGateway implements PaymentGatewayInterface
+{
+    public function initiate(PaymentRequestData $data): PaymentResultData
+    {
+        return new PaymentResultData(status: PaymentStatus::Pending, gatewayTransactionId: 'fake-txn', gatewayPayload: []);
+    }
+
+    public function verify(string $gatewayTransactionId): PaymentResultData
+    {
+        return new PaymentResultData(status: PaymentStatus::Completed, gatewayTransactionId: $gatewayTransactionId, gatewayPayload: []);
+    }
+
+    public function refund(string $gatewayTransactionId, string $amount, string $reason): RefundResultData
+    {
+        throw new RuntimeException('not needed for this test');
+    }
+}
+
+test('the factory resolves KbzMiniAppGateway for the KbzMiniApp method by default', function () {
+    $gateway = app(PaymentGatewayFactory::class)->make(PaymentMethod::KbzMiniApp);
+
+    expect($gateway)->toBeInstanceOf(KbzMiniAppGateway::class);
+});
+
+test('registering a fake gateway swaps the resolved implementation with no call-site changes', function () {
+    $factory = app(PaymentGatewayFactory::class);
+    $factory->register(PaymentMethod::KbzMiniApp, FakePaymentGateway::class);
+
+    // A call site that only knows about the factory + interface, never the
+    // concrete gateway class — this is exactly what an Action/controller does.
+    $callSite = fn (PaymentGatewayFactory $factory, PaymentMethod $method): PaymentGatewayInterface => $factory->make($method);
+
+    expect($callSite($factory, PaymentMethod::KbzMiniApp))->toBeInstanceOf(FakePaymentGateway::class);
+});
+
+test('the factory throws when no gateway is registered for a method', function () {
+    $factory = new PaymentGatewayFactory;
+
+    expect(fn () => $factory->make(PaymentMethod::KbzMiniApp))->toThrow(RuntimeException::class);
+});
+
+test('the factory is bound as a singleton', function () {
+    expect(app(PaymentGatewayFactory::class))->toBe(app(PaymentGatewayFactory::class));
+});
diff --git a/app-modules/payment/tests/Unit/KbzSignatureTest.php b/app-modules/payment/tests/Unit/KbzSignatureTest.php
new file mode 100644
index 0000000..54c191e
--- /dev/null
+++ b/app-modules/payment/tests/Unit/KbzSignatureTest.php
@@ -0,0 +1,60 @@
+<?php
+
+use Modules\Payment\Support\KbzSignature;
+
+/**
+ * Fixture derived by running bnf_event's own `KBZPay::joinKeyVal`/`signature`
+ * (App\Strategies\Payments\KBZPay) against this exact params array and key —
+ * this pins the port to the old code's actual behavior (domain.md §6).
+ */
+$fixtureParams = [
+    'timestamp' => '1700000000',
+    'method' => 'kbz.payment.precreate',
+    'notify_url' => 'https://example.com/api/v1/webhooks/kbz',
+    'nonce_str' => 'fixture-nonce',
+    'version' => '1.0',
+    'biz_content' => [
+        'appid' => 'APPID123',
+        'merch_code' => 'MERCH001',
+        'merch_order_id' => 'EVB-FIXTURE-001',
+        'trade_type' => 'MINIAPP',
+        'total_amount' => '15000',
+        'trans_currency' => 'MMK',
+        'callback_info' => 'urlencode',
+    ],
+];
+$fixtureKey = 'test-merchant-key';
+
+test('joinKeyVal flattens nested biz_content and sorts keys, matching the old algorithm', function () use ($fixtureParams) {
+    expect(KbzSignature::joinKeyVal($fixtureParams))->toBe(
+        'appid=APPID123&callback_info=urlencode&merch_code=MERCH001&merch_order_id=EVB-FIXTURE-001'
+        .'&method=kbz.payment.precreate&nonce_str=fixture-nonce&notify_url=https://example.com/api/v1/webhooks/kbz'
+        .'&timestamp=1700000000&total_amount=15000&trade_type=MINIAPP&trans_currency=MMK&version=1.0'
+    );
+});
+
+test('sign matches the known fixture hash produced by the old KBZPay::signature', function () use ($fixtureParams, $fixtureKey) {
+    expect(KbzSignature::sign($fixtureParams, $fixtureKey))
+        ->toBe('29F95FB3DCCEC866A68A07E9A1B25BEEE9F355529B5D37395A04B2282FC48BB1');
+});
+
+test('sign is uppercase SHA-256 and stable for the same input', function () use ($fixtureParams, $fixtureKey) {
+    $signature = KbzSignature::sign($fixtureParams, $fixtureKey);
+
+    expect($signature)->toBe(strtoupper($signature))
+        ->and($signature)->toHaveLength(64)
+        ->and(KbzSignature::sign($fixtureParams, $fixtureKey))->toBe($signature);
+});
+
+test('sign and joinKeyVal ignore any pre-existing sign/sign_type values', function () use ($fixtureParams, $fixtureKey) {
+    $withStaleSign = [...$fixtureParams, 'sign' => 'stale', 'sign_type' => 'SHA256'];
+
+    expect(KbzSignature::sign($withStaleSign, $fixtureKey))
+        ->toBe(KbzSignature::sign($fixtureParams, $fixtureKey));
+});
+
+test('joinKeyVal drops null and empty-string values', function () {
+    $params = ['a' => 'x', 'b' => null, 'c' => '', 'd' => '   '];
+
+    expect(KbzSignature::joinKeyVal($params))->toBe('a=x');
+});
diff --git a/app-modules/payment/tests/Unit/PaymentServiceTest.php b/app-modules/payment/tests/Unit/PaymentServiceTest.php
new file mode 100644
index 0000000..767dfae
--- /dev/null
+++ b/app-modules/payment/tests/Unit/PaymentServiceTest.php
@@ -0,0 +1,60 @@
+<?php
+
+use Modules\Payment\Contracts\PaymentGatewayInterface;
+use Modules\Payment\Data\PaymentRequestData;
+use Modules\Payment\Data\PaymentResultData;
+use Modules\Payment\Data\RefundResultData;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Enums\RefundStatus;
+use Modules\Payment\Factories\PaymentGatewayFactory;
+use Modules\Payment\Services\PaymentService;
+
+test('initiate resolves the gateway for the request method and delegates to its initiate()', function () {
+    $gateway = Mockery::mock(PaymentGatewayInterface::class);
+    $data = new PaymentRequestData(
+        bookingId: 1,
+        merchantOrderId: 'EVB-001',
+        amount: '15000',
+        currency: 'MMK',
+        method: PaymentMethod::KbzMiniApp,
+    );
+    $expected = new PaymentResultData(status: PaymentStatus::Pending, gatewayTransactionId: 'EVB-001', gatewayPayload: []);
+
+    $gateway->shouldReceive('initiate')->once()->with($data)->andReturn($expected);
+
+    $factory = Mockery::mock(PaymentGatewayFactory::class);
+    $factory->shouldReceive('make')->once()->with(PaymentMethod::KbzMiniApp)->andReturn($gateway);
+
+    $result = (new PaymentService($factory))->initiate($data);
+
+    expect($result)->toBe($expected);
+});
+
+test('verify resolves the gateway for the given method and delegates to its verify()', function () {
+    $gateway = Mockery::mock(PaymentGatewayInterface::class);
+    $expected = new PaymentResultData(status: PaymentStatus::Completed, gatewayTransactionId: 'EVB-001', gatewayPayload: []);
+
+    $gateway->shouldReceive('verify')->once()->with('EVB-001')->andReturn($expected);
+
+    $factory = Mockery::mock(PaymentGatewayFactory::class);
+    $factory->shouldReceive('make')->once()->with(PaymentMethod::KbzMiniApp)->andReturn($gateway);
+
+    $result = (new PaymentService($factory))->verify(PaymentMethod::KbzMiniApp, 'EVB-001');
+
+    expect($result)->toBe($expected);
+});
+
+test('refund resolves the gateway for the given method and delegates to its refund()', function () {
+    $gateway = Mockery::mock(PaymentGatewayInterface::class);
+    $expected = new RefundResultData(status: RefundStatus::Completed, gatewayRefundId: 'REFUND-1', gatewayPayload: []);
+
+    $gateway->shouldReceive('refund')->once()->with('EVB-001', '8000', 'customer request')->andReturn($expected);
+
+    $factory = Mockery::mock(PaymentGatewayFactory::class);
+    $factory->shouldReceive('make')->once()->with(PaymentMethod::KbzMiniApp)->andReturn($gateway);
+
+    $result = (new PaymentService($factory))->refund(PaymentMethod::KbzMiniApp, 'EVB-001', '8000', 'customer request');
+
+    expect($result)->toBe($expected);
+});
diff --git a/config/services.php b/config/services.php
index ee75030..57389e4 100644
--- a/config/services.php
+++ b/config/services.php
@@ -36,6 +36,7 @@ return [
     ],
 
     'kbz' => [
+        'app_id' => env('KBZ_APP_ID'),
         'merchant_code' => env('KBZ_MERCHANT_CODE'),
         'merchant_key' => env('KBZ_MERCHANT_KEY'),
         'base_url' => env('KBZ_BASE_URL'),