Official Google Shopping Content .NET SDK by Google. It contains API Key access and OAuth 2.0 access. In addition, this resources shows web applications, service accounts, and installed applications with the aim to facilitate API interaction in .NET language.
Blippy and Swipely both launched fairly recently with the hopes of making spending a social experience. Both services are very similar in that they both allow you to connect your bank and credit card info, as well as other accounts like iTunes and Amazon. After you've set everything up, as you start spending, you have the option to share these purchases with everyone. And now you can extend that sharing with the Blippy API.