I was encountering a fatal error when initializing Unibill on Amazon Kindle. Specifically, it realted to the class "com.amazon.inapp.purchasing.ResponseReceiver" not being found. I hit it running the latest version of Unibill from the Asset Store, and Unity v4.5.3f3.
Well, I just figured out that our AndroidManifest in project file had an old Amazon IAP v1.0 receiver path. The path is changed for v2.0. The manifest receive must be "com.amazon.device.iap.ResponseReceiver" - not the path listed above.
Look out for this, anyone carrying a project from Amazon IAP 1.0 to 2.0!
The issue produces a fatal crash and requires console log output to diagnose.