I have used this service quite often, when traveling to the Swan. I must admit, though, that I always arrive during daytime hours.
If you purchase ahead, online, there is a 10% discount for using promo code WEB10. The roundtrip fare is always much less, per ride, than the one-way fare.
If you purchase online, you will still need to get your boarding pass and receipt from the Mears desk, which is on level one, just past the car rental counters. Mears has a location on both the A & B sides. So, after you collect any checked bags, you can go down to level one, have your confirmation number handy, and print off your Mears boarding pass and receipt. There is always a Mears rep at the desk when I have been there, for any assistance.
After printing off from the kiosk, you exit the terminal and report to the outside Mears desk. They will give you a pager, and you can sit outside at the curb to be called for your ride. Every once in awhile, there were not many other pax waiting for rides, so I have been put in a Mears taxi, instead of a shuttle van/mini bus.
It's not possible to say in advance if there will be other stops, prior to yours. My daytime experience in the shuttle is that the Swan and Dolphin will be last. The actual ride can be about one hour long, with other stops ahead of yours. They will tell you that the wait to board a shuttle will be about 20 minutes, after checking in at the curbside desk.
The driver will store your bags and give them back at your stop. A tip is customary.
If I don't have a rental car for my Swan stays, I use the Mears shared shuttle.