We're having problems with the Sandbox Server URL for the App Store Server Notifications. Unfortunately, we're not getting any callbacks from Apple Server while making in app purchase.
We examined the URL endpoint and verified that it complies with Apple's guidelines, as well as appropriately configuring it. Ref. https://help.apple.com/app-store-connect/#/dev0067a330b
Please review the types of notifications supported with v1 or v2. Server Notifications are only sent for events where its applicable to receive a notification to your server. For example when a customer purchases a non-consumables IAP's, there is no notification sent as there is no action to take server side as the purchase must occur in-app or be redeemed in-app, where there will be a successful (unfinished) transaction in the customers queue. BUT if there is a refund of that purchase, that does occur outside of your app and may require server side action, thus a REFUND notification type is sent. https://developer.apple.com/documentation/appstoreservernotifications