Refund failures may be due to:
- INVALID_PARAMS: Specifying an amount larger than the refundable amount
- CANCELED_USER: Refunded user has already canceled
- REFUND_LIMIT_EXCEEDED: Maximum number of refunds exceeded
- REFUND_WINDOW_EXCEED: Exceeded refund period
- INTERNAL_SERVER_ERROR: An internal error occurred on the PayPay side
- Example: Incorrectly setting merchantPaymentId for paymentId
- RESOURCE_NOT_FOUND: You specified a paymentId that does not exist