Вы можете передать любые дополнительные пользовательские данные для идентификации вашего платежа в параметре trow на стороне сервера:
payment.setCustomID («некоторые дополнительные невидимые данные вы можете передать, и они будут возвращены в URL обратного вызова ipn на ваш сервер»);
// Create a basic PayPalPayment.
PayPalPayment payment = new PayPalPayment();
// Sets the currency type for this payment.
payment.setCurrencyType(currency_type);
// Sets the recipient for the payment. This can also be a phone number.
payment.setRecipient(paypal_recipient);
// Sets the amount of the payment, not including tax and shipping amounts.
payment.setSubtotal(new BigDecimal(currency_cost));
// Sets the payment type. This can be PAYMENT_TYPE_GOODS, PAYMENT_TYPE_SERVICE, PAYMENT_TYPE_PERSONAL, or PAYMENT_TYPE_NONE.
payment.setPaymentType(PayPal.PAYMENT_TYPE_GOODS);
// Sets the Custom ID. This is any ID that you would like to have associated with the payment.
payment.setCustomID(transaction_id);
// Sets the Instant Payment Notification url. This url will be hit by the PayPal server upon completion of the payment.
payment.setIpnUrl(payment_url_callback);
// Sets the memo. This memo will be part of the notification sent by PayPal to the necessary parties.
payment.setDescription(product_description);