I'm using version 3 of the in-app billing API. I have a single, managed, non-consumable item. I have not released this feature in my app yet, so I want to decide on the purchase payload contents before there are any purchases.
From "Security Best Practices":
Set the developer payload string when making purchase requests
With the In-app Billing Version 3 API, you can include a 'developer payload' string token when sending your purchase request to Google Play. Typically, this is used to pass in a string token that uniquely identifies this purchase request. If you specify a string value, Google Play returns this string along with the purchase response. Subsequently, when you make queries about this purchase, Google Play returns this string together with the purchase details.
You should pass in a string token that helps your application to identify the user who made the purchase, so that you can later verify that this is a legitimate purchase by that user. For consumable items, you can use a randomly generated string, but for non-consumable items you should use a string that uniquely identifies the user.
When you get back the response from Google Play, make sure to verify that the developer payload string matches the token that you sent previously with the purchase request. As a further security precaution, you should perform the verification on your own secure server.
Rightly or wrongly, I have decided not to take the "further security precaution" of setting up a server to perform purchase verification. And I do not store my own record of the purchase -- I always call the billing API. So is there really any reason for me to do this payload verification? The verification API itself certainly verifies the identity of a user before reporting an item as purchased, and if an attacker has compromised a device (either the app or the google play API), I don't see any benefit of doing an additional check on the user's identify on the device where it can easily be circumvented. Or is there a reason to do this that I'm not thinking of?