Amazon oops!

This morning Amazon started a promotion, selling the new Tran-Siberian Orchestra album "Night Castle" for cheap. I bought it using the credit card number Amazon had thoughtfully (and un-asked for, as far as I can remember) remembered from the last time I bought something, then I downloaded the MP3s.

A couple hours later, I got a notice saying that the credit card I had used was invalid and my order was being canceled. I went and checked my Amazon account, and sure enough the credit card was an old one. However, I've already downloaded the MP3s.

Do I call them up and say, "Hey, I need to give you money!" Do I just re-buy the album using a valid credit card even though I don't need to download it again? Why didn't they run the credit card before allowing me to download the album in the first place?