IPn

IPN stands for Instant Payment Notification in the context of online payments. It is a server-to-server message format used by payment processors such as PayPal to notify a merchant's system about events related to a transaction. When a customer completes a payment, the processor sends an HTTP POST request to a preconfigured URL on the merchant's site, including data fields such as txn_id, payment_status, mc_gross, mc_currency, payer_email, and a custom field chosen by the merchant. The merchant typically validates the message by sending the entire payload back to the processor with an extra parameter cmd=_notify-validate; the processor replies with VERIFIED or INVALID to confirm authenticity. On receipt, the merchant can update order status, trigger fulfillments, or adjust inventories and accounting.

IPN messages are asynchronous and may be delivered multiple times if delivery fails or the processor retries; therefore, merchants should implement idempotent processing and maintain reconciliation and logs. Security considerations include using HTTPS, validating that the message came from the trusted processor, and not exposing endpoints to untrusted sources. IPN is widely used to automate order processing, refunds, chargebacks, and subscription events, enabling real-time updates without direct user interaction.

Beyond payments, the acronym IPN may be used in other domains, but Instant Payment Notification is the most common reference in e-commerce and digital payments. See also: webhooks and message verification.