G2A Pay asks for a IPN Notification URL, what is it?

The IPN notification URL is required to receive notifications to WHMCS to mark invoices as paid. The URL to the G2A Pay callback handler is WHMCS_ROOT/modules/gateways/callback/g2apay.php

Make sure to replace WHMCS_ROOT with your WHMCS installation URL. For example, ours is https://billing.aspnix.com