Microsoft allows 3rd party in-app purchase payment providers.
Here is an paypal example [url removed, login to view] [url removed, login to view] .
Here is an user experience [url removed, login to view]
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".
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.