Did you realize about that? if user go to buy for first time, no problem. Well... the only problem is as you akready know.. that you receive the payment but user didn't receive the product (for dowloadable products). In my case, my solution is to send a gift card to the customer so they can buy it again. Paying with GC they don't log in or create the account so the system never can send an Zen Cart invoice.
But today I found a new bug... if you pick a product to buy (you will see the GC payment option, of course... before the payments options page (THAT IS THE BIG MISTAKE)... But.. if you proceed to login... after logged the GoogleCheckout option disappear. Magic!!!!

LOL... for these problems is because the most perfect solution is Paypal. I can believe the most recent version is 2007.