I can't find any way either on Stardock or this site to modify my "cart", and yet, it seems I have a quantity of 2 electronic-DL preorders for Galactic Civilizations II. This probably happened because I clicked on the "Buy This" button on two separate occasions and it seems to have stored the previous occasion.
While I did wish to preorder it, I'd rather not pay 84$ for the honor of doing so twice. Note that my browser is Firefox, but the problem seems to just be that there's no function available on the entire site for managing the cart. It's a bit frustrating since I've been following this game for awhile and I can't seem to find any way whatsoever to access my "cart", which is a feature I'm used to taking for granted on any site that sells anything.