“You can always return your Zipcar early, as long as it has at least 1/4 tank of gas. Since you technically have the car for the full time, you won’t get any money back for returning a Zipcar early as another member can’t rent it until the end of your reservation.
You do, however, want to avoid returning a car late as Zipcar’s late fees begin at $50. If you think you’ll be late while on your trip, you can see if it’s possible to extend your reservation. Due to the late fees, overestimating your time is better than getting stuck with late fees.”