You used to be able to get free Amazon MP3 credit for choosing standard shipping instead of prime, not sure if that's still around.
Also for buying digital content (movies, game downloads etc.) they sometimes throw in extra credits.
In June I checked my credit there and had $15 in software/video games, didn't need anything then so left it "for next time". Needed something a month later and credits were all gone. Apparently credits have an expiration date.