Directly in your application code where you are using the
Google Payment API to handle the user's in-app purchases (IAP), simply insert and use the 3rd-party's API (e.g
Slack) to send you a notification when the IAP has been successfully processed.
Since this is your own application that is running on App Engine, you have access to directly code in notifications as described above and there is no need to create any hack or webhook.
Note: Google Groups is meant for general product discussions and not for technical support. If you require further technical assistance with implementing the above, you can refer to
Stack Overflow and
Slack Support.