Here's some insight from WIMCO, one of the largest, if not the largest, villa rental companies on St Barth. Nobody knows what policies SXM will implement, but this is something to consider.
Even with SXM closed, the SBH airport opens to US travelers on June 22. Tradewind Aviation operates 8-passenger shared charters between SJU and SBH, a shuttle service if you will. Cost-wise, think of a R/T ticket on the shared charters as being about $1200 for the one-hour flight.
https://www.wimco.com/vacation-trav...arget=/st-barts-covid-19-testing-center/The above procedures obviously work much better for someone renting from a villa rental agency than they do for someone renting via VRBO or Airbnb.
At the end of the day, if you're diagnosed with COVID during your stay, there's no way out of the French islands. That's also something to consider.
