I can confirm a similar issue, which I’m currently discussing on How to handle IAP subscription - #11 by Moses (scroll down to the newer comments)
I’m almost done creating the Game Maker extension, but cannot properly test it through Amazon’s App Tester.
The following simply does not work - as I can see the verbose logs of the Appstore SDK is in Release mode.
adb shell setprop debug.amazon.sandboxmode debug
Waiting for a reply on the post for now. Did you manage the solve that?