Seats on Thomson flights

[Trailblazerram]

I have been on a cruise in the Caribbean before and cannot remember this being the case but it has been suggested that we can book our seats prior to flying, a common enough practice I know. However as this is a charter flight we will be charged £15 per seat per head. Why was this not explained and quoted in the cruise price. We are saving the airline time by booking the seats ourselves but they are asking us to arrive and check in 4 hours before take off, and charging us for doing it. Is this normal procedure?

Also the travel agent have sent measurements for cabin bags that are less than our normal carry on luggage (about 2 cm too large in all directions) but when we have been on the Thomson website the bag size is fine. Who do we believe?

I am tempted just to turn up like we do for normal flights and risk being apart for the trip rather than pay this money. What do other people think?