Update:
I returned my rental car at the Miami Downtown HLE location and parked the car at the garage indicated by Cordelli. (BTW - a mail truck was in the exact spot as shown on google map) The Hertz shuttle driver took my bags and loaded them onto the van. I proceeded inside to the rental counter and close out my rental. I was at the port within 20 minutes. But keep in mind that during the low season (summer) the process will be easy and quick. It may be the opposite during winter months when there are more ships at port.
:-: GOLD STAR to Hertz for this extra service. :-: