In Android, you can have several Pub/Sub so it is not a big deal to add multiple S2S URLs. Apple appstore provides a single space for adding a URL for listening for S2S notifications.
If you replace your S2S url with our Purchasely S2S URL at the respective store, in order to get your backend system updated about subscription events of existing users from App Store, we have developed “S2S Forwardings” integration.
This will redirect raw Apple’s S2S messages to the referenced URL. You should provide a reference to all the products and plans in Purchasely because we need it to forward the messages to your application.
Then the messages are sent directly to your URL. Incase your server doesn't respond, we have a retry strategy to send the update to your backend.