Our family will be getting to Sanford around midnight. Will I still be able to get a car when I arrive without waiting until morning?

I would definitely schedule ahead, but I have rented at Sanford around that time.

I rented a car online prior to our midnight arrival. It was apparent the agent was only obligated to stay until the last reservation was fulfilled.

You may have to schedule the car ahead of time. The rental location may not be staffed at that time.