if the return is IN MCO (all the major players are) IME there are folks there to receive you 24x7
IME if there is not an attendant there is a drop box for the keys and they mail (email) the final bill. Have done this many times with NO problems.
I'm local to MCO and ALWAYS try to be IN THE TERMINAL with 2 hours before the flight and this assumes carry on only AND TSA PRE.