There are several rental car companies at the Pier where you arrive. I really don't think a prior ressie is necessary, as they have quite a few cars to offer. The local companies, vs. Avis & Budget had better rates on the same cars.
If you do make a ressie ahead of time, beware of Avis- confirm your rate beforehand. Make sure you have all of your confirmation paperwork with you, as you may get a surprise when you turn in like we did. It is a different reservation website than their national one, which is very good. When we went to return the car, the bill was double. We presented our paperwork to show what rate we reserved for & they honored it. If we didn't have the proof with us, I think we would have been stuck.
The nice part of renting a car on St. Martin is you can get out of Phillipsburg & go to the french side. They drive on the right in St. Martin too, unlike St. Thomas. We visited the Butterfly Farm, went to Orient Beach for about 2 1/2 hours ( watch the sun there!), had lunch, shopped in Marigot & went to that crazy airport beach & watched the planes land directly above our heads. We were back in plenty of time to shower & leisurely get ready for dinner. It was great!
