Ok now I could make it work, but it was a stony way 😛
Here some things that are important:
1) For the implementation use App Tester first, if this works go in the developer console to "Live App Testing" (LAT)
2) Before you start LAT, be sure you added your in-app items and wait until they have status "Live"
3) On the Android Manifest, there is a mistake in the guide, you have to move android:permission = "com.amazon.inapp.purchasing.Permission.NOTIFY" from the <action> tag into the <receiver> tag.
4) For testing with LAT you could upload a debug.apk to use the logs
So long, good luck and Merry Xmas 🙂