When you buy something from amazon, you can input the code at the checkout (right before you finalize ur order) and it will deduct $20 or however much ur promotional code was.
I just sent uncharted 3 and arkham city. It should be there some time next week. They're both like brand new. Didn't even use any of the codes inside. Keep an eye out!
I enjoyed Vanquished a lot. Do I think its worth $60? No. But only because to me, there's no replay value. On the otherhand, the unique and energetic gameplay was enough to keep me intrigued throughout the game. I got it for $26 from Amazon warehouse and it was definitely worth it.