It looks like Amazon is pretty excited about the release of hearthstone on its Android app store. So excited, that they are looking to help new and veteran players get card packs on a discount.
If you download hearthstone from the Amazon app store when you purchase card packs, you purchase them through Amazon. This gives Amazon the ability to give you kickbacks, such as rewarding your account with Amazon Coins. These coins can be used just like cash in the app store and with in-app purchases.
So for every $20 you spend in hearthstone on card packs, they will give you 30% back in the form of Amazon Coins. Amazon coins have a value of about 1¢ to 1 coin. This means that when you spend $20, you will then get 600 coins, or $6.00 to spend on more card packs.
Remember, these cards are tied to your Battle.net account, so you could essentially buy your cards for sale on your phone, then continue to play the game on whatever device you choose. Not a bad deal if I say so myself. Be sure to check out Amazon's official page on the matter.