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I have been trying to implement In App Payments for almost a week now, constantly working few hours here and there just to find another bug that needs to be taken care of. The Problem with implementing In App Payments is, that is in't easily tested.
Using a real device comes with its own struggles. You have to be plugged in. And yada yada yada. Anyways I have managed to get it working. At the beginning I skipped the Part of verifying the receipt which is required so that nobody can "scam" the system and get your virtual items for free. I skipped it for the beginning because I have encountered an annoying Error, early on.
I was so happy to get everything working and running that I even created a StackOverflow Question only to Answer It myself. I was searching here and there for this simple answer. Now I got it myself. You can check it out here: