Basic rules I find are...
Charter or Budget = you pay for ski/board/boot carriage.
Scheduled flights = you don't pay extra.
Generally.
What I would say is ignore what Thomas Cook et al say, especially about pre booking, unless you know it's their own charter flight or a budget like RyanAir or EasyJet.
If it turns out the flight is a scheduled by BA or whoever, then just look on the airline's site. But usually it's your normal 23kg allowance or whatever for hold bag, plus you get to take a ski bag and boot bag extra. In this case, just turn up at the airport with the bags.
If it's a budget airline, check the airline first for fees as the tour operator might charge extra for pre-booking over the airline's fees.
I should add that I have never I believe had a ski bag weighed, and taking a bigger bag with more items (two skis, a board, etc) is no problem. A good way with the cheapos to get someone else's gear in. Another tip is if you are thinking of changing skis out there, a bigger bag means you can carry two pairs back. Customs are non the wiser

(though a good tip is to post your receipts back rather than carry them in the luggage).
Also, if you're taking skis etc and paying for them, consider a scheduled flight instead. e.g. RyanAir costs with taxes + baggage costs comes out just about the same as a scheduled BA flight, plus with BA you get free food and more comfortable seats.
P.S. Hand luggage is the one bag plus now an extra for laptops etc. Ensure you have valuable gadgets in the hand luggage, and don't store lithium batteries in the hold. I just chuck chargers and stuff in the hold with clothes but all things of value (other than skis) go in the hand luggage.
Oh and finally, with skis and boards you will be spending extra time waiting at Oversized Baggage for your stuff to come off the plain. Worth considering if time is an issue when getting out of the airport.