I use the latest IAP SDK (2.0.76) and followed all the instructions. However there is an issue which must be on Amazon side. After the user accepted the purchase and entered his password, a 2FA / MFA website opens. With the remote it is possible to enter the code, but it is not possible to press the LOGIN button, even if it has focus. I can reproduce it with my Fire TV devices (latest devices with all Fire OS updates applied). A lot of my customers reported this and cannot purchase my apps.