If you have to travel anymore than 1-2 hrs, require a deposit, which will be put toward the donation. If you are leaving the state, especially for only one person, require at least 50% deposit, & they should be covering your travel expenses as well.
This just happened to myself as well... I was heading to one state, but stopped in another on my way, as 2 had pre-booked. Drove through a blizzard 9 hours the first day, & 7 the next, to ensure I would be on time...only to receive a text 3 hours before the appt time, "Got called into work, can't make it"... lol. It was a 3 hour appt, which accounted for a large chunk of why I decided to stop. Thankfully, the other, a regular, still showed up & actually booked an extra hour
I would say, if you already have the flight & hotel paid for, you minus as well go... otherwise you are completely just throwing the $ you already spent down the drain. You should also require deposits from now on, & have a disclaimer on your website explaining everything.