We have regular events of 200+ pax where we need to shuttle them from the airport to the event location. For convenience of the participants and their flight times, we offer 2 different shuttle times (not negotiable is the word). The bus company that handles the transfers needs to know the total number of seats ahead of time, as it's a 2+ hour ride and not a constant pendle.
We have buy links for the fares, and then a separate form in which people can indicate their preferred shuttle time. The problem is people change their plans and hence ask us to change their shuttle times. It halfway works but it's messy because people are not reliably responsive to forms, especially when we utilize multiple different ones. They also have to pay for the shuttle separately from their main ticket (and separate from the form with the times), adding another variable and more confusion.
How can I simplify this, make it more intuitive with less steps and friction, while still keeping the two different times option? For anybody who has a similar event setup, how do you do it?