My understanding is Princess does offer a transfer program as Rick refers to, where they will print boarding passes and airline luggage tags on the ship. However I can't find it anywhere on the Princess website - which frankly has one of the poorest FAQ sections I've ever seen.
If Princess is like Royal Caribbean with this service, they do not offer it online but rather will place a form in your cabin which you complete with your airline info, number of bags, etc., and you turn this in at Guest Relations. Isn't it called 'EZ Check'?
It is something not offered at every port, and also not all airlines participate. Also, they have a limit on the number of people allowed to use it and some are rejected.