I have actually almost already finished this project for another client(who is not paying up front), and I retained full rights to redistribute and resell my software. It would take very little to modify it to your liking.
It operates on a framework pre-made to handle all the billing and login, so it will pass shopify's review process easily.
I cost more per hour, but the work is also 99% done. I prefer equity/hybrid deals, but I can guarantee a better, faster product than anyone else available.
I would like to negotiate a price(or share of future rev/profit) for the work already done, but I can show you up front what you would be getting.
Thanks,
Mike
248-227-2465