add notes/remark and refactor round-trip

app-modules/payment/src/Actions/InitiatePaymentAction.php

@@ -43,6 +43,10 @@ class InitiatePaymentAction
             throw PaymentInitiationNotAllowedException::notPendingPayment($booking);
         }
 
+        if ($booking->is_return_leg) {
+            throw PaymentInitiationNotAllowedException::isReturnLeg($booking);
+        }
+
         return DB::transaction(function () use ($booking, $method) {
             $booking = Booking::whereKey($booking->id)->lockForUpdate()->first();
 
@@ -62,11 +66,12 @@ class InitiatePaymentAction
             }
 
             $merchantOrderId = $this->merchantOrderId($booking);
+            $amount = $this->amount($booking);
 
             $result = $this->paymentService->initiate(new PaymentRequestData(
                 bookingId: $booking->id,
                 merchantOrderId: $merchantOrderId,
-                amount: (string) $booking->price,
+                amount: $amount,
                 currency: self::CURRENCY,
                 method: $method,
                 notifyUrl: $this->notifyUrl($booking, $method),
@@ -76,7 +81,7 @@ class InitiatePaymentAction
                 'booking_id' => $booking->id,
                 'gateway' => $method,
                 'status' => $result->status,
-                'amount' => $booking->price,
+                'amount' => $amount,
                 'currency' => self::CURRENCY,
                 'gateway_transaction_id' => $result->gatewayTransactionId ?? $merchantOrderId,
                 'gateway_payload' => $result->gatewayPayload,
@@ -85,6 +90,20 @@ class InitiatePaymentAction
         });
     }
 
+    /**
+     * A round trip's payment is combined on the outbound leg — covers both
+     * legs' price, since the return leg never gets its own Payment
+     * (domain.md §2b). A plain one-way booking just pays its own price.
+     */
+    private function amount(Booking $booking): string
+    {
+        if ($booking->linked_booking_id === null) {
+            return (string) $booking->price;
+        }
+
+        return bcadd((string) $booking->price, (string) $booking->linkedBooking->price, 2);
+    }
+
     /**
      * A booking can have more than one payment attempt (retry after
      * failure), so the merchant order id must be unique per attempt, not