looking for flight advice between SJU & STX

$120 is about the best you are going to do for round-trip fares. Caribbean Sun beats the price sometimes... but there have been a lot of complaints about missed/delayed/cancelled flights and also about not being able to make connections on time either to or from Caribbean Sun and winding up stranded.

For private flights, most aircraft will hold 5-8 passengers. If you are traveling with a large enough group to fill the airplane, the per person cost might be a little lower than the commercial flight fare. The main benefit is you get to choose your flight time rather than going with what's offered by an airline. That can make it possible to connect to a flight that you otherwise could not or it might avoid an extra long layover, etc. Private charters are usually priced per flight, not per person. You pay the same fare whether you fill one passenger seat or all available seats with people in your party. On private flights, you can also sometimes take pets that the airlines will not transport. Most small aircraft have baggage and weight limitations that are more restrictive than some of the larger commercial aircraft.

I looked into Cape Air for SJU to STT and their flights were comparable - $120, less if you book earlier. I was looking at a somewhat last minute trip