I have been asking the same question, hope you can get some answers. I think it is open to all, call Ship Services and they can answer questions. I am more concerned about the safety of our luggage and it getting where it belongs!
This service was renamed "Luggage Direct" and is no longer SEBS. It only costs $19 per person and you can use it out of Seattle if you are flying out of SEA at 11:30AM or later. I highly recommend this service. You get your boarding passes in your room the night before you disembark. You then get off the ship first as the cruise line handles takes your luggage for you to the airport. You can enjoy your last day without having to lug around bags...it's great!