I had to spend some Amazon coins, so I bought an app and specified coins as the payment type. I had one cent in digital credits and that one cent was used. The order details shows the digital credit as coins. I guess their devs don't know the difference between coins and digital credits? Why even bother to ask how I want to pay for something if you're not going to use it. And you can't know how much you paid for something until you have already paid? That should be illegal.
