When you drop off a car at the Rental Car Center at MIA, especially with one of the major rental car companies, you'll find a staff of greeters who will scan the code on the door, close your rental and print/email your receipt. You can then be on your way to the port via taxi or rideshare.
When you return your car to a city/neighborhood location, you typically have to park your car and go inside to close your rental. Inside, on a port day (which is pretty much every day in Port Miami, you'll find the above-described chaos with so many coming off ships and renting cars while you and others return cars. If you have a hotel, then taxi/rideshare/hotel shuttle to the hotel. In either case, save the heartache and return to the airport rental car center.