Right, here's the problem. If I follow your tutorial and install: https://github.com/poiuytrez/AndroidInAppBilling
I get this when trying to build with XDK...
Plugin "com.smartmobilesoftware.androidinappbilling" (3.0.2) installed.
Plugin "cordova-plugin-legacy-whitelist" (1.1.0) installed.
Plugin "cc.fovea.plugins.inapppurchase" (3.4.1) installed.
Error: Plugin com.mcm.plugins.androidinappbilling failed to install.
Which, for over a year now, you've been saying is an XDK issue.
So, correct me if I'm mistaken in this logic but C2 is telling XDK to add that plugin via the XML and you're saying add the other plugin manually and any time you add the two to an XDK project it won't build them. It seems to me that it's the choice of plugins that C2 is telling XDK to use that's causing the issue.
I've been ripping export .xml files apart and even adding both manually, XDK will build with either but not both.