Welcome to the Forums, and thanks for posting! Quick answer: at the moment, the Appstore SDK Plugin for Unity (and specifically its libAmazonIapV2Bridge.so) is 32-bit only, which means that you would need to build and submit your game/app as a 32-bit binary.
If, however, you require your game to be 64-bit, then I would suggest to use Unity’s IAP directly, please see:
Oh, but that basically means that using the official SDK makes the app incompatible with the latest Amazon devices (such as Fire tablets 2021 and later), quite a strange situation. It would be good if the documentation specified that it is better to use Unity IAP, this would save developers a lot of time.