Amazon recently released their in-app purchase system to us devs, so the Amazon version of the game can finally be brought in line with the Google one. An update including all fixes to date, plus the Campaign 2 in-app purchase is now in submission, and should be with you in a few days.
In other news Sentinel 3 is part of the Because We May
indie promotion, and has price dropped on Android, iOS and Mac. If you've been on the fence now's the time to grab it