It would depend on the airline. Excess baggage charges avry quite a lot. if you have to change flights, these usually puts the cost up too,Also if you know you are going to be over, some allow you to purchase additional baggage allowance in advance at a cheaper rate and what they would charge you at the gate. Please also be careful that most enforce a mx of 32kg per case regardless of how much extra you have bought and some have reduced this to 23kg, so you may have to take another bag.
Once you know which airline, call them and check it out with them.That way you can decide in advance how much you want to take and won't get any nasty surprises at the airport.
Princess Cruises generally use a Charter Flight to get passengers to the ship so if you are not going earlier on a standard flight you will need to ask that question to Princess. If you are flying out on scheduled airlines it will depend on the airline and here I would suggest that you go online and look at the small print for that airline because it can be as much a £7 per kilo for excess baggage. Hope that helps .... Neil.
We have been informed the luggage allowance is 23 kilos.