You gotta share the eBay gift card listing that works with the code, I was gonna buy a Best Buy gift card with the code but it's not letting me so this would be a great alternative.
EDIT: So you just buy something that works with the code and add the eBay GC to the cart?
I sent an email to Best Buy because I can't call them right now to see if they could apply the discount to the Map Pack 1 DLC even if I pre-ordered the Special Edition. Hope it works out!
EDIT: No go, figured as much. ;(