If you get fewer takers than you hoped for

It can be difficult if you have collected deposits on the expectation of taking, say, 30 passengers in your group, and you finish up with only 25.  Most operators, ourselves included, will do what they can to help, but sometimes it will not be possible to absorb all the extra cost.

My suggestion is to ask for prices for 35, 30 and 25 passengers so that you know from the outset what the price will be for different group sizes.

I would go further and tell them the price on the assumption of 25 passengers, and then if you do get the 30 you originally hoped for you can either reduce the price or give some added value, like an extra guided visit or an extra inclusive meal.