I've never used google checkout from the merchant side, so I just poked around for a bit. I'll avoid doing the whole lmgtfy thing, but this appears to be what you're looking for:
Content policies seems to be the most-likely damning.
Mentions of donations elsewhere are heavily paired with "donate" buttons, and I don't know if they had used such a button. Cost + voluntary bonus does, to me, imply valid use of the word "donate", and I'm hoping for something concrete, given the number of authoritative-sounding "can't say 'donate' w/o 501.c3" comments.
http://checkout.google.com/support/sell/bin/answer.py?answer...
There's also this page, which I found by just clicking "content policies" under the merchant section (look at "fundraising"):
https://checkout.google.com/support/sell/bin/answer.py?answe...