A customer yelling at you about a coupon usually isn’t about you. Bad days get handed down the line.

Somebody shouting at a counter is usually carrying something that has nothing to do with you: a bad diagnosis, a worse manager, a day that went wrong before they arrived. You are simply the first person they are allowed to be angry at. Knowing that does not make it pleasant, and it does keep you from taking a stranger’s bad week home.

Stay level, fix what you can fix, and get a supervisor for the rest. Afterwards, say out loud to a colleague that it was not about you, because naming it helps it pass.