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 to a customer for a failed refund, use this factual base: confirm the order, charge, booking, or service problem and separate verified facts from what is still being checked.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer the available refund, replacement, correction, or escalation with dates and ownership. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not use empathy as a substitute for a remedy or ask the customer to repeat information you already have. Move payment, privacy, accessibility, and product safety incidents into the documented specialist process immediately. You do not need the other person to agree with a boundary for you to follow it consistently.