How is the new fuel surcharge which starts in Feb going to be paid? By adding onto the cabin charges?

You'll have to pay it before your docs are released. It won't be added to the onboard account.

Wait! I am wrong. For Princess cruises that are already paid in full they indeed will add the fuel surcharge to your shipboard account.

We're going on Jan 19 th so we beat them this time but it'll be a surprise for some people in the near future.

It shouldn't be a surprise, RichC; I have received notifications from Princess concerning my two upcoming cruises.

What will be a surprise is if the price of fuel goes down and they discontinue the surcharge.