We usually stay at Mt. Vernon. Last trip we had no car for the entire 10 days. It's easy to call a taxi from the little shop at Mt. Vernon or many of the restaurants or bars along the beach might call one for you. It was about 50USD round trip to Marigot. That was a few years ago, so maybe more now.
I definitely agree with Carol, make arrangements with Wilfred brooks.
We love to simply find our spot on orient and relax--no driving around or itineraries.