I'm going to go a bit against the prevailing advice, such as don't book a round trip, and suggest that you look into the Moorings transfer package. We used it last year and I assume it is still in place. For $90 a person we got transfers from STT to the ferry landing, the round trip ferry ticket (not counting baggage and other fees that I assume are standard), transfers from Road Town ferry dock to the Moorings, and the reverse on our return.

Someone was waiting for us at STT with a Mooring sign, and other than a groan when he saw our luggage, got us all assembled and to the dock. We had a bit of a wait in Road Town until the driver collected all the inbound passengers, but that happens when you fly to EIS.

On our return our son was planning to meet his girlfriend at STT and the two of them catch a cab to the St. John ferry in Red Hook. Her flight was delayed so he headed to Red Hook on his own. They honored his ferry to STT voucher for the much longer ferry to Red Hook ride.

All in all in was a painless process that we'll probably use again in our 07/2020 trip.

One word of warning to all ferry riders. Seas can be a bit rough to be sure to take you sea sickness meds before boarding.