Thanks for the suggestion guys.. No word from the client how they're going to proceed at the moment. A second window as a popup will probably be the choice at the moment, the trick is just making it work
Basically calling the cart in the background (possibly even in a small 100pixel frame at the top the screen to enter the product to the cart. Then once the user is ready to check out, launch that frame in full screen. That MIGHT take care of it. I'm concerned about dropping sessions etc too...
Still batting around ideas. I'll let y'all know what direction we diecide to head with this client. Any other suggestions, keep 'em comming!