If it were me I'd give it away, the few dollars it's worth wouldn't make much difference to most people. The competition idea sounds fair, since you're going to get several people who aren't in a position to obtain it otherwise, and I'm sure they'll all have their respective sob stories.
I say you go to giving/trading forum and make a nice contest (like funny stories or who can beat you in a online match or have some people trade you the code for cards) that way its fair and fun and nobody can complain.
Oh and make it for non-US and JP players, its unfair that we got the not only now but early and everyone else only got extra cards and still doesn't have it in the eshop.
Hm, I would've gotten a code as well since I have the applicable games to register with NoE to get it for free... however, NoE doesn't allow northern european countries to participate Then I will not buy it even though I want it.
You are lucky you got an extra, I say you should either trade it with someone or sell it... however you won't get much back, just a few bucks, but still.. you can buy an extra cup of coffee sometime