Penny, nothing has changed at Princess; the shore excursions go on your onboard account and your card isn't charged until the day you disembark. What is a little confusing is they put a hold on a certain amonut of money on your debit or credit card; twice now we thought we were overcharged, but within a couple of days everything was set right.
Here is cancellation policy.
Other than tours involving hotels, flights or private vehicles, the "closing time" is usually, as you say, 24 hours before ahead. Although, I have heard of people getting refunds if they cancel on the day of the tour due to illness and get validation from the ship's medical center.