WillM
I do believe (unless you booked your SJU to EIS leg through American and SEA to EIS is one booking) that you will have to pick up your luggage at baggage claim, go check in at Cape Air, check your luggage again and go through security. We go Saturday so will let you know but this is what we had to do last year.
Definitely post your experience. I'm especially interested in whether you need a PCR test and travel authorization in SJU in order to claim your bags and check in to your connecting flight.
