On the SXM official website it says the following with regard to SLS..

St. Maarten Laboratory Services
Address 1G Hamster Drive
Hours of Operation 6 Days a Week
Monday – Friday 7:300 AM – 11:00 AM (PCR) – 1:00 PM (ANT)
Saturday 7:30 AM – 9:00 AM (PCR) – 10:00 AM (ANT)
Phone +1721-542-2518
Email prescreening@sls.sx
Website www.sls.sx
Make your Appointment Walk-ins
Type of Test PCR & Antigen Tests
Cost of Test PCR $75 USD | Antigen $45 USD

Looks like it is walk-ins only. Others have reported waiting in line up to two hours. Doesn't sound good to me.