While these do work, please note that Gamestop loss prevention will delete/ban your PowerUpRewards account a week or so after you use these. You will need to make a new account at another store and then complete the purchase, or use someone else's. Just FYI.
I can vouch for this guy as well. I just purchased in invite from him for $25 and it worked great! I was able to download a bunch of keys right away, and I am trying out the Azure credit now.