I wanted this Google API version 2.5 .NET sample code for a long time and finally built it myself:
http://www.capprime.com/software_development_weblog/2010/11/29/UsingTheGoogleCheckout25APIWithASPNETMVCTheMissingSample.aspx
If you need classic WebForms instead of MVC, let me know.
I didn't include samples of what the packets look like, because honestly, it doesn't matter (the API is supposed to wrap the packets). It's not a lot of work to modify the sample in a couple of places and have it send you an e-mail with that data.