I know this doesn’t directly answer your question, but we opted to just bring our seat from home and check it at the airport. The cost of buying a travel bag was less than renting the seat, and we didn’t have to worry about quality or cleanliness. We’ll probably do the same in December.
My son has rented a car seat a few times. This past June, I believe it was a $5/day charge but we might of did some negotiating to get that rate because we were renting 2 cars for several weeks. The car seats are about as clean as the rental cars are. Sunscreen and sand does a job on the upholstery. His child was almost 2 years old. If you are bringing an infant, I might recommend bringing your own seat. I don't know the car seat laws in SXM.