Name the impact without exaggerating it
Describe who had to wait, worry, redo work, spend money, or make a choice with incomplete information. For a message about how to apologize for overspending a shared trip budget, use this factual base: confirm the booking, timing, guest, transport, or budget change and who now has to adjust.
Make repair practical
Offer the repair you can actually complete, with ownership, timing, and any limit stated honestly. 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.
Let future behavior carry the proof
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. A promise inside a message is weaker than a visible change in what you do after it.