I've just built an iOS app that uses a non-reoccuring subscription model.

You hit "buy" and then when the client device receives the receipt from the Apple server it whizzes it off to my server to validate it and add the subscription to my user database. If the connection fails before my server has responded then the request is cached, ready to be resent the next time the client comes online.

My problem is, what happens if the connection fails between when the user hits the buy button and the receipt is received by the client from Apple? Theres no way for it to know that the purchase has been made?

Presumably I should add a "restore purchases" button that sends all of the users receipts to my server which checks if they've already been validated and their respective subscriptions added to the database? Is there a less intrusive way of doing this?