From here http://developer.android.com/google/play/billing/api.html :
Managed In-app Products
Managed in-app products are items that have their ownership information tracked and managed by Google Play. When a user purchases a managed in-app item, Google Play stores the purchase information for each item on a per-user basis. This enables you to later query Google Play at any time to restore the state of the items a specific user has purchased. This information is persistent on the Google Play servers even if the user uninstalls the application or if they change devices.
What you want to do is ( like in the example app ) query the inventory on successful setup of your in app service and consume any managed item thus granting access to it.