Thorsten, can you fly out of Fort Lauderdale? According to Google Flights, there are flights in December to Beef Island for well under $200 on Jet Blue. You won't have to handle your baggage and it's just a short taxi to Trellis for the ferry.

I feel you on going through STT, which is why we don't do it. Your other option is the afternoon Cape Air flight from STT to EIS for $175 in December.