if i were in this situation i would create a small app showing just that this is only a "plugin-app" for the main app and in mainapp i would use the package-manager to search for this special "addon". If it is installed then i would enable function x in mainapp.
I havent tried this but that´s what i would try. in app billing like erel suggested may be the better idea.
When the guru @
Erel says "You can use in app billing and disable features based on the purchased products." then it MUST be an good idea