However, the kindle does not play well with this game (Game of War: Fire Age) and several times while I was attempting to make an in game purchase with coins, the coins got deducted, but the items I purchased never appeared.
Most in-app purchases tend to have similar denominations to the amazon coins (, , , , 0, etc.), so before sales tax, buying amazon coins gives you just enough currency to make IAPs (with perhaps a few cents left over).
I also unfortunately bought the largest denomination possible because the description says they can be sent as a gift and I thought I would be able to transfer some of the coins to another Amazon account--I'm a US citizen living in Japan, and some apps require an jp login while others require the one. It also turned out to be quite a lot of work just to buy the coins and use them in any apps due to my issues with having registered shipping addresses and billing addresses in two regions.
And then with the in-game taxes, instead of saving money it ended up costing me more than if I just hadn't bothered.
To spend Amazon Coins, please use a Fire Tablet or TV, or you can install the Amazon Appstore on your Android device.
You can even continue the games you’ve already started.