This can happen sometimes when you have a last-minute mechanical problem, or if the unexpected weather makes the trip unsafe. Please note that you are not expected to complete a trip in these circumstances. We anticipate that weather or mechanical problems may be an issue, and the passenger is expected to have a back-up plan. Driving or buying a commercial ticket is definitely considered "above and beyond," but is much appreciated by the passenger.
If you decide to drive a mission or buy a commercial ticket for a passenger instead of flying, you can make a note of this in the mission report. You will enter the cost of the commercial ticket you purchased, and that amount will be recorded as a donation.