What a shocker, the Amazon Appstore is now three years old. To celebrate the third birthday of the Appstore, Amazon is listing a few apps as free, but they are apparently focusing on “coin-back” in-app purchases. Think of these purchases like a cash-back card: you purchase an IAP in an app, but Amazon will refund you 50% of the purchase in the form of Amazon Coins, which can then be used inside of their own store.