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Microsoft allows 3rd party in-app purchase payment providers.
Here is an paypal example https://github.com/paypal/Windows8SDK http://paypal.github.io/Windows8SDK/ .
Here is an user experience http://msdn.microsoft.com/en-us/library/windows/apps/hh924350.aspx
So we need SDK like PayPal but with one exception: Insted of paypal page we just need fake HTML page with 2 buttons "Pay" and "Cancel".
Additional Project Description:
04/11/2013 at 12:20 PDT
We need SDK that allows app developers use 3rd party payment gateway.
SDK is should consist of only 3-5 methods.
And also no payment gateway need to be integrated, just fake 1 page HTML with 2 buttons on it.