It depends on the platform you use and the availability of applications for specific platforms. If you've purchased the application and it's not available for Kindle Fire (Android OS), then you should ask for a refund. But since the payment was complete, the developer should give you all the instructions to download the apps. The Kindle Fire is an Android device, which means it works with APK files. You should request the APK files from the developer.
It depends on the platform you use and the availability of applications for specific platforms. If you've purchased the application and it's not available for Kindle Fire (Android OS), then you should ask for a refund. But since the payment was complete, the developer should give you all the instructions to download the apps. The Kindle Fire is an Android device, which means it works with APK files. You should request the APK files from the developer.