I suppose I was just frustrated, took 2 months to solve. But we did get it working in the end... So +1 to that.
I'd like to see a C3 based example project in C3 Editor, that'd be great. Also with detailed steps and links, screenshots of what your settings inside the Itunes connect should look like.
I suppose the error should really come back from Apple: "Warning, your IAP will not work because: Tax information incomplete".
But that's apple for you....
Anyway, I've verified both IAP work on Android and iOS. I'm super happy with C3, it's the best platform in the world I think.