I don't know how long this code is good for, but you can go to the Amazon MP3 store and enter code FALLMP3S for $2 in MP3 downloads. What do you want with your credit? There are a ton of really good whole album downloads for only $5, and this awesome Patti Smith album is today's $3.99 deal.
You could also get almost 3 of the 69 cents singles. Check out that list HERE.