If it is only for a day I doubt you would be penalized. I would call and check.
When they charge for an early return is when people say they want a weekly rental for the lower charges and return it before the week is up. Then the rate goes back to a daily rate. You are only renting for a day so it shouldn't be a problem.
I would check the rates and make a reservation ahead of time. Sometimes online rates are cheaper than walk ups.