Not sure about sales and stuff, but bestbuy.com also sells $20 and $50 codes, haven't ordered from there before, that's where I'll order next time, since for whatever reason gamestop charges me sales tax on the codes even though they aren't supposed to.
Not that I'd buy skins of any kind that only change the look ever, but don't they pretty much have to be on disc or in a title update or compatibility pack or something or no one else online would be able to see them??
At first you did get rewardzone points when using a gift card, then they changed it to where the person buying the gift card got the points. not sure if that's still the case.