State the minimum cost of taking part
A clear invitation might say “The exhibition is free. Return bus fare is $6 per person. Bring lunch if you like; the café afterward is optional.” The required outlay for someone using that bus is $6. A $9 café lunch would make their personal total $15, but it is not a condition of joining.
Give people a private way to choose
Offer the plan and its amounts without asking anyone to name what they can afford in front of others. Let adults choose the exhibition only, the meal too, or a different outing. Avoid calling the cheaper route the “budget option” when a neutral description such as “meet at the entrance” will do.
Check costs at the moment the group commits
If a discount requires the whole group to commit, check everyone is comfortable before booking. Avoid collecting money for optional items as if they were required. Give the amount and payment timing in the invitation so no one has to ask in front of others.
If a discounted group ticket needs six people but only four have confirmed, calculate the cost for four and offer it again. A person who expected a $6 outing should not discover that they must buy a $15 ticket at the entrance. Update the invitation when the required cost changes, even if only one part of the plan changed.