Hi there,
Just to let you know do not make the same mistake as me by heading to the car rental desks at the port, don't the AVIS it's not there. It's a further five minute walk.
Head up the multi coloured sidewalk and head for the colourful "Welcome to St Maarten" and then exit the security barrier and turn left to the town. The rental station is on the left and clearly marked about a 3 minute walk from the security gate. If you come back and the AVIS rental is closed the hand the keys into the bar next door, having first taken pictures of the rental car to show the conddition you returned it in (a little trick I learned)
I rented last time from there less than 3 weeks ago on the WBTA