tl;dr - Use any browser, visit https://gaming.amazon.com/home to get the redeemable codes, and avoid the App itself
There is only one reason to use the Amazon Games App: any full game marked with the Crown icon. Literally everything else is a GOG, Epic or Steam code. Even then, something unique may show up as exclusive that can be worked-around. (A Monkey Island title was exclusive, so I downloaded it and launched it in SCUMM instead)