This is a pretty solvable problem from a car service's POV: overprovision your fleet to account for people who want to take a car for a full day or even a week, and then charge significantly less than ZipCar but more than a local ride-hop (you could even change pricing plans so you pay by the hour/day instead of by the mile). Because your fixed vehicle costs are amortized over a much larger number of customers than ZipCar and your car can get itself from where there's supply to where there's demand, you can eliminate the reservation aspect and charge significantly less.

On an economic level, the occasional person who wants to reserve a car for a weekend uses up a much smaller fraction of a car than one who owns one outright.