Hi, I've solved this issue for Android:
In the config.xml remove this line:
...Instead add this:
<plugin name="cc.fovea.cordova.purchase" spec="~7.1.0">
<variable name="BILLING_KEY" value="YOURBILLINGKEYHERE" />
...you don't need to bundle that plugin with your zip file for Phonegap Build as it already uses it. The problem seemed to be that Phonegap Build no longer supports the com.mcm.plugins.androidinappbilling plugin.