Princess used to have a “grace” period of five days after final payment is due but quite a few years ago, they eliminated that. Final payment is due the date they say and the system will automatically cancel the booking. It’s on every statement or invoice I’ve received. (My TA sends me the Princess invoice and then her invoice since she always saves me money.)
Travel agents almost always ask their customers to authorize final payment 3-5 days before the Princess final payment date. That’s in case there’s a glitch, credit card error, charge doesn’t go through, error in giving the TA your credit card, etc.
The bottom line is that there’s always a final payment date and the responsibility is always the passenger’s to authorize and make final payment on time. Doesn’t matter whether you book directly or with a TA.