I have picked up an Alamo rental at the MIA car rental center after cruises each of the last two Januaries, driving to WDW and dropping off the car at the Dolphin. Each one-day rental was less than $65 including taxes and plus gas and tolls, booked on costcotravel.com. The Miami-area toll roads use toll-by-plate and once you’ve incurred a toll Alamo charges a $3.95 service fee for the 24-hour day plus the cost of the tolls, billed to your credit card on file sometime in the month to six weeks after the trip. In 2017, we took a taxi from the port to the car rental center. Cost was about $35 including tip and much more convenient than wrestling with luggage to take the Alamo shuttle or the
DCL bus to the airport (which then requires taking the light rail from the airport to the car rental center). This year, I used Lyft at a cost of about $18 including tip. For both the taxi and the Lyft pickup, the porter who assisted us with our luggage took us right to the vehicle and was tipped accordingly.
Rental rates were the same for a full 24-hour day as for anything over 4 hours when I booked, so we kept the car overnight and dropped it off the following morning at the Dolphin.