I will be on the POA on 10/24 and booked all my car rentals through Alamo on each island. I did it right on line. they will help you choose the right location and send a shuttle to meet you at the pier. They were very accomodating and they had great prices.
Here is a travel agent suggestion - book your cars on a Tuesday or Wednesday through the website. Prices are lower than if you did the same booking on a weekend.