Where do you enter the code? I don't see a line for it on even the screen where you enter your credit card number.
And yeah, you have to be careful as many of these aren't deals even at 40% off-but some are. Halo 3 for $18, Halo Wars, Gears, etc. for $12ish. Some very solid deals, and it looks like shipping is $5 regardless.
EDIT: I mean regardless of how many games you buy, apparently, so it's pretty cheap if you're buying a few.