We always use Budget or Avis so I can't tell you abt National. But the rental companies are all in an area a short ride from the airport terminal within the confines of the airport "campus"& shuttle busses take you back & forth.
We have rented from both Alamo and National in Ft. Lauderdale post cruise. Both have shuttle buses that make the rounds of the cruise port on a regular basis. We did not need to call them. They come about every 20 minutes. All of the major car rental companies provide this service. We rent from the company that has the best price.