I think the Dollar pick up is at the airport, or in the vicinity of the airport. send them an e-mail and ask where they are located. the best way to get there by taxi. you should be able to pick one up right off the ferry pier. Probably around $8 from there to the airport.
ISIS only carries, sub-compacts. so if you want a small car they are good.
We used Ace this past trip, had a very nice 2014 Toyota Rav4. They are also at the airport. Some people have reported being over charged by Ace, but that was not our case. We will be using them again on our next trip.