The customer at the window didn’t cause your bad day. Don’t hand it to them with their change.

Moods look for somewhere to land, and the nearest stranger is the cheapest target because they will not argue back and you will never see them again. That is also what makes it corrosive, since you are practising being short with people who did nothing, and the practice sticks. The feeling is real and the aim is wrong. Separating those two is most of what being decent at work means.

When you notice the day turning, name it to yourself before the next customer arrives. Take a breath at the window and keep your voice level. If you do snap at somebody, apologise in the same minute.