went into a gamestop and the guy said if I buy a 3 month xbox live membership I should get a code for an additional 3 month membership free. I ordered two - 3 month memberships. sure enough 2 additonal 3 month memberships popped up on the receipt.
first step - redeem each code on the receipt. (6 months total)
second step - redeem 1 card and add auto renewal (extra month free, 4 months total)
third step - go to pc and turn off auto renewal
fourth step - redeem 2nd card and add auto renewal (extra month free, 4 months total)
I paid 24.99 for each 3 month card, 49.98 total and ended up with 14 months of xbox live.
I dont know if it works at every gamestop but I live in california and it worked here.
the associate said he doesn't even know if it still works until the transactions complete and it spits out a receipt. tried it out and it worked today!!