In the Sub PurchaseCompleted you save the purchase result in a logfile, at the next start you have to check that the logfile exists, if not, then you call the method "MyStore.RestoreTransactions".
Next problem is if the user has deleted the app, then there is no more logfile.
Dan would be better, except the logfile, also to have a database on the internet, with which you can check if the user has purchased the app.
But, where should I know the user if it is not possible to check the unique user id?
Here another strategy must be applied.
I do it this way: before the user uses the "in-app purchase", they must authenticate themselves with a Google account. So I have the user ID and can save it to my database.