There is another alternative. I usually book my own air, often at prices lower than what the line charges. The worst case is if the line has to cancel my cruise and I have to pay a change fee to apply the cost of the ticket to another trip, but I'm way ahead even if that does happen. I usually buy the airport transfers through Princess, so they meet my flight just as though I had booked my air throuh them.
If you don't quite trust yourself to book your own air, your travel agent can do so. Most travel agencies seem to waive their surcharges for issuing airline tickets if you book the flights as part of a cruise or resort package.