I bought the game but it doesn't give me an option for Android download 

You have bought Normal version, Android is 13.99 cause it have Deluxe Walkthrough integrated.

Oh, ok. It won't let me pay an additional $2 to make up the difference. Is my only option to re-buy it? 

I can't refund games, only itch can do it. Not sure if a 2$ donation would be good for you to have the good version...