https://developer.amazon.com/docs/in-app-purchasing/iap-testing-overview.html says that there are two ways to test IAP: with the App Tester and with DevTest.
Both of these approaches require the Amazon Appstore to be installed on the current device. App Tester would work if I could just download an App Tester APK and sideload it, but the App Tester is only available from the Amazon Appstore.
https://community.amazondeveloper.com/t/how-do-i-test-my-app-emulator-beta-test-iap-physical-device/1779 explains how to test an app on an emulator, but not how to test IAP.
How do I test my IAP on an emulator?