Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to apologize for missing shared transportation, use this factual base: confirm the booking, timing, guest, transport, or budget change and who now has to adjust.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. 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.
Respond to escalation safely
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. You do not need the other person to agree with a boundary for you to follow it consistently.