The following is my June and September experience at SXM. Others may have had a wonderful time, or maybe a worse time than I did.
Arriving you will be put on a bus and driven the 150ft+/- to the big white tents. When you enter the tent, and you have carry-on only, immediately go to the right lane. In June there was a person standing there giving directions, I told them I was in-transit, no baggage to claim, St Barts Commuter, and they pointed to what is now considered the in-transit desk, which was just inside the tent to the right. I use St. Barts Commuter, their "desk" was just there in front of me and also another person asking me "which airline"... They gave me my boarding pass and pointed across the room to a door, which I went through. It put me in the large tent waiting room where I found "gate3" and waited for my SBC flight. Which in June, left early. In September I used Tradewinds to get to SBH from SJU. My feeling is that heading down to SBH is a much better experience than returning to SXM to fly home.
Returning from SBH to SXM in June and September was, about as much fun as a sharp stick in the eye. Off of St. Barts Commuter into the tent and there was no going through in-transit, even though I had no luggage and ticket in hand. Forced to go through SXM customs/immigration and out of the secure area. From there I walked (with everybody else) around the outside of the tents to the right to an outside check-in area, which was just an area with tables and airport employees directing you back inside the big tent to repeat check in, security, another boarding pass, and back into the big tent/boarding area/packed with people waiting area. This check in and security screen can take a while. There are several places in the tent to buy a cold drink and a sandwich. The tent can get VERY full, and rather warm.
Before Irma, SXM was no real issue in-transit. Going to take some time to get back to that.
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