Responding to an IPN message
With each successful notification that you receive, configure your endpoint to send Amazon a "200 OK" response immediately after receipt. If you send another response, Amazon SNS performs retries every hour for 14 days. If your server can't be reached when the IPN is sent or when you respond with a 4xx, there will be no retries.
Note: Amazon processes your payment requests in an asynchronous manner. Usually each notification is delivered to each of your specified endpoints exactly once, but the distributed nature of Amazon SNS and transient network conditions cause an occasional duplicate message. Be sure to design your application so that if duplicate IPN messages are received, your payment workflow doesn't break.
For more information about the format of the Instant Notification, see HTTP/HTTPS Notification JSON Format in the Amazon SNS Getting Started Guide, which is available from the Amazon AWS Documentation portal.
The contents of the NotificationData member that is returned in the Instant Notification are described by the following publicly available XSD: https://amazonpayments.s3.amazonaws.com/documents/payments_ipn.xsd.
All IPN messages received from Amazon are signed. For more information about how to verify the signature, see Verifying the Signatures of Amazon SNS Messages in the Amazon SNS Getting Started Guide.