I think you got free credit (is the credit called "coins"?) from buying/downloading other apps on the Amazon App Store. They usually have this kind of promotion going every once in a while and it's a great way to get some apps for free.
I use their app store a lot as it doesn't give me the headaches the Google Play store, and it's really nice because it offers many free apps, but I guess a downside is that many developers do not update the Amazon versions of their apps because of Amazon's requirements.
If you want to spend that free credit, buy the Sonic games. They are fantastic ports, remastered in widescreen, and they usually have extra content which wasn't accessible in the original games. However, I would hold off from buying Sonic the Hedgehog 2, as Amazon gives that game for free every once in a while.