I don't believe there are any rental locations right at the pier. There are several at close by hotels.
But if you're only going to the airport to leave I'd just take a taxi. It's a set rate for the taxi's. With tip will only cost $25 or $26.... and that's per taxi, not per person.
Since we live in Florida, we plan to rent a van and drive to Port Canaveral and return it there. Then have a van booked to drive home. Does anyone know the closest car rental to the port or someone you have had good service with. Also do the rental companies pick you up at the port?