Doesn't the Kindle Android app allow you to buy ebooks and subscriptions?
Also,while Microsoft does support in-app purchases in their store, they don't restrict other payment methods like Apple does. From Microsoft's Store guidelines:
http://blogs.msdn.com/b/windowsstore/archive/2012/07/20/maki...
Using your own billing system
Your app and service may already depend on a particular transaction provider or benefit from ties to other lines of business. Your customers want the trust and efficiency of a familiar, trusted transaction experience. You can use your own transaction provider within your app to provide the experience your customers expect.
If you are not using the Windows Store as your transaction provider, you will want to make sure that your app meets all of the certification requirements such as:
Identifying the transaction provider to the user during purchase confirmation
Prompt the user for authentication before processing the transaction
Your payment processor must meet the current PCI Data Security Standard