Same thing happened to me. My wife purchased a $50 gift card from Best Buy and it has been in a card on our Xmas tree for weeks. I opened it scratched off the code and entered it and it said already used. Contacted Sony CS just to be told the code was redeemed on the 18th. Unreal.. The tell me to ask for a replacement from BestBuy, which as we know is going to laugh at me when I show up with a used card. So my wife paid $50 for a piece of cardboard.
Update: I learned that my Mother-in-Law got the cards from a BestBuy in North Carolina. They are the cardboard cards, not the plastic ones. So... buyer beware of Best Buy?