Google Wallet for digital goods provides easy integration with a single API. Merchants include the Google Wallet for digital goods library in a web page where digital or virtual goods will be sold. You can designate any part of your web page to start the purchase flow.
If you are developing an Android application, please take advantage of the in-app billing features available on the Android Market. More information is available at http://developer.android.com/guide/market/billing/index.html.