SneakyShort version: You get Amazon coins from "buying" 9 free apps, which earns you enough coins to buy GTA:SA that is currently on sale. Purchasing GTA earns you another like $20 worth of coins that you can then spend on whatever you want.
Guess you need at least one android device on your account. Sadly I do not have one. Aww.
EDIT: Guess be trying out Bluestacks (Android emulator on PC) ages ago counts as an Android device! Ill try this out.
EDI2: Yup, worked! I've got 2,201 coins now. I can spend 699 coins for Minecraft Pocket Edition and get 350 coins back. Also Monument Valley is pretty cheap. Of course I don't have a way to play them....yet.