Is there anything you think you were doing that may be interpreted as fraud?
Now, I am using PayPal's authorize-and-settle function. I don't settle the transaction until I see in my logs that the user has downloaded the item. If more than a few hours has passed then I email the customer and ask them if they are having some kind of problem with the download.
(no particular point to this, just thought the story might be interesting to others.)
EDIT: And we had maybe 10 orders go through Google Checkout, max (thus the $200ish amount). The other 470 or so went thru PayPal.
Still, the issue is that Google provides no way of finding out the cause.
Do what briansmith suggests, you just can't be too cautious.