I used to wait to book our end-of-cruise transfer while on board so I could use OBCs. Our last Princess cruise (ending in NY) wouldn't add us to the transfer list - CS rep on board said they were "having trouble with the transfer/bus company and were not doing any further business with them." Mind you, this was a couple days before arriving in NY. Not a relaxing way to end a cruise! At the time, we had never used Uber/Lyft, so were really panicked. Stranded at the pier!! Woe and fear.
We ended up just joining the transfer queue, intending to see if we could pay the ticket taker. No one checked tickets! We climbed on the bus - relieved but feeling a bit guilty (but not for Princess!). We gave the driver a nice tip once we got to the airport, tho!