I'm impressed with how "on it" your company is. Keep it up.
Unfortunately I'm having another problem. I did search to see if anyone else had this prob and didn't find anything similar, so here goes. If you go to http://spankingmachinemusic.com you can recreate the problem probably. Site is currently in sandbox mode. I added both products to test the checkout process and proceeded to check out. After entering all my information and hitting checkout, the page gives me pink boxes saying I need to enter the information, which I swear I did. (It also says to "choose desired payment method" below the credit card information. I can probably fix that myself, but thought you'd like to know.) I'm running WP 3.1.2, using authorize.net and have entered the correct keys.
Hopefully this is as easy as the other problem ; )