Purchases from iTunes and Google Play have to be made in their payment systems. What we provide is a receipt validation service, so that you can secure the transaction (it's trivially easy to cheat this, if the client just checks if the receipt is valid, and rewards the item based upon that). Have a look at this blog post for more information on this: https://blog.playfab.com/blog/show-me-money-receipt-validation-ios-and-android/
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