In-app purchasing coming to Windows Phone 8, not before
Example of in-app purchasing for Windows Phone
In-app purchasing allows users to switch from a free to a paid version of a game, make purchases in a game for extra weapons or turns. For example, with the popular Draw Something game, users can purchase "bombs" for additional words to draw or to help solve a tough word. Users are charged on the same credit card they have set up to purchase apps with. For developers, it gives them a way to up-sell customers and grab more revenue with each app.
source: TheVerge via PCMag
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Things that are NOT allowed: