No credits needed. You just need an account with Amazon.
Steps:
1. go to this link:
Link: http://www.amazon.com/b/ref=sv_dmusic_3?ie=UTF8&node=2258933011
2. select Category you want to download
3. sort by -> Price: Low to High
4. you will see FREE in red on the right side of each song.
5. preview and/or download the song you want.
