Working retail teaches you that a calm voice can end a customer’s meltdown faster than any policy.

Escalation needs a partner. Someone arriving angry is looking, usually without knowing it, for a person to match them, and a steady voice leaves nothing to push against, so the whole thing loses its shape within a minute. None of this means being a doormat, and you are allowed to end a conversation. It means calm is the more effective tool, not only the nicer one.

Slow your speech down instead of speeding up when someone is shouting. Say what you can do rather than what you cannot. Learning this at a counter gives you something you will use everywhere else.