[jgannon]

I had this problem today, too. Just keep trying to add it to your cart, and it'll work eventually. When it says that it's invalid, go back, click in the quantity box, and hit Enter. If it doesn't go anywhere, then you should have one in your cart. (And yes, I had trouble with it in Firefox 1.5.)

Edit: It's still doing it for me, for both big and little CIMs. Sometimes clicking works, sometimes Enter works.