Here is the post that explains this on OO.
Gamestop hadn't shipped my order yet (or even charged me) this morning (10th), so I canceled it (you have to email them..how stupid is that). Then, I just bought it through Steam. I really want the map, but I want to play the game more. So.. yeah. AND THEN, I get two emails in succession a few minutes ago. First, the package has been shipped. Then, an email saying they can't cancel my order because it shipped already. And it seemed automated. I'd swear they did that on purpose. I hate Gamestop.
Already have the code. I will beat the best price for the game currently by $5 (whatever it happens to be). PM me and I can send you my paypal address. I can then PM you the code. I've done trades on OO in the past, not many though. And never here. I did a LOT of trading on Home of the Underdogs back in the day (which of course no longer exists in anything but bastardized copies). But I can link you to my ebay feedback if necessary.