Instant Payment Notifications (IPN’s) will be fired in the following cases when the merchant has provided a NotificationURL in their getTransaction or doTransaction API callIPNs) are sent back to the NotificationURL in (but not limited to) the following scenarios:
When a payment on the PayU redirect either fails or is successful When a user session times out on the PayU redirect with no chance for the user to finish payment(3D secure redirect)
When transaction pending review for fraud is either approved or rejected by case managers. In the case of approval the IPN will fire after the payment has been attempted and will return that result.
The NotificationURL can be set to an HTTP or HTTPS endpoint. PayU will attempt to post the IPN 3 times with 1 minute intervals, failing which the IPN data will automatically be sent to the merchant as an email.
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For a detailed overview of the process please go to the Instant Payment Notification section here