Is there an Avis location at the port? I haven't been to Miami, but I know there is at the Vancouver and Sewark ports. In FL, though, there are NO one-way charges for rentals due to state laws! So you should be able to just drop it off without penalties at the port (if there's an Avis there) without worrying about a taxi. Doing a rental in the opposite direction might also work on the way back if it's cheaper than a taxi to MIA.
Enjoy your vacation!