Well, not to be harsh, but I think you might be shit out of luck. But you can test your luck:

1) Contact Amazon and see if you can get a refund/exchange since you haven't used the code. I've used their customer support multiple times, and they were very understanding.
2) Contact SE to see if you can trade like Skyyman said.
3) Post on game trading sites, here, or craiglist and see if someone will trade you (if you wanna take that risk).

Aside from that, you may have to accept your lost and buy a new key.