I just threw together an extremely quick and dirty example of how the Notification_Center works (in a desktop app since the module is pure Xojo and works on any platform 
). You can find it here:
https://www.dropbox.com/s/am7zl44x7df0zbz/notification_example.xojo_binary_project?dl=1
For StoreKit + Notification_Center, there should be an event with “name” SKProvideContentForProductNotification (a constant defined in the InAppPurchaseHelper class) that is triggered when a product is purchased/restored. As long as you have subscribed to that notification name, you’ll receive a notification with a userDict containing a single key of “productID” and a value of the ID string for the item that was purchased. Hopefully that makes sense