I think I may have figured it out on my own. For some reason, Internet Explorer 8 isn't showing the "apply coupon" button to the right of the entry field. I tried it in Firefox, and now I can see the "apply coupon" button. So although I had been entering my code previously, I couldn't see the button to apply it! It's entirely possible that this is a unique problem, as I've been having video issues (specifically flash and directx) on my system for some time now. I thought I had them sorted out, but perhaps not.
Anyhow, once you let me know what's going to happen with the copy of "Out from Boneville" that's still in "My Games", I'll try the order again through firefox, and it should work fine.