The android billing plugin is broken in release build. Everything works perfectly in the debug build though.
The method ServiceConnection.BillingHandler.BuyProduct() does nothing in release build.
I also tested the included InAppBillingTest project which has the same problem so the issue is not specific to my project.
I have the latest Xamarin since December 22nd 2016.
Does the Android billing module need some additional project changes for release build?