Shmoo is right. You can take a free shuttle provided by a number of rental car companies from the port to their office (a mile or two away), and then get a one way rental for the day, and then drop it off right at the airport. Most rental car companies in Orlando don't charge extra for the one-way rental. Shop around. You should be able to find a good deal. We used Budget because it had the type car we wanted for a good price for that day. Reserve it ahead of time, but you don't have to pay anything until you pick it up.