In a recent purchase from Amazon there was a large postcard sized advertisement for this deal. I went on the 25th and registered and to receive the $15 gift certificate you have to mail in the postcard sized advertisement (I think they call it your special offer postcard) and a 3X5 card with your info.
I think this will only work if you make a purchase from amazon and receive one of these postcards in your packing slip though I might be wrong.