Write from facts rather than momentum
Pause long enough to remove accusations, assumptions, and details that belong to someone else. For a message about how to apologize for neglecting a travel task, use this factual base: confirm the booking, timing, guest, transport, or budget change and who now has to adjust.
Offer a proportionate next step
Match the size of the request to the relationship and to what the recipient can reasonably decide now. Offer a replacement plan, reimbursement, or clear decision before the next real deadline. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Protect privacy on both sides
Do not minimize lost money or time, and do not invite extra people into a plan without consent. Follow venue, transport, insurance, and local safety rules when an emergency or stranded traveler is involved. Use the most private legitimate channel available and do not share the exchange to recruit support or pressure.