WP8 App Payments

I am currently making a Windows Phone 8 silverlight app. In the app, we would like to allow the user to pay via paypal or another payment method. I also looked at payments in apps from the dev center, but couldn't see the variable value payment there.

Any help on what to use or examples on how to do this would be greatly appreciated.

Update: Per the Windows Phone Store policy, you cannot use paypal, for example, and can use Windows Phone in your apps.