Cheating the system

SO when the 3DS was announced at Amazon, you could trade in a DS lite for 50$ which I did. Now when I sent it in the giftcard for the 3DS wasn't applied to my gift card balance, so I emailed them and told them to fix that, which they did,

It hit me a few minuites ago that the promotional credit I randomly got was the DS trade in getting applied, so I got the credit applied twice.

Now this is just a guess, I am not sure, but still cheating the system is an awesome feeling.