The rude cashier might be on hour nine. Notice the shift, not just the attitude.

Behaviour comes out of circumstances you cannot see, and the length of somebody’s day is one of the big ones. Nine hours in, with sore feet and a queue that has not ended, most people are a worse version of themselves. That does not make rudeness pleasant to receive. It does make it easier to not take personally and not return.

Try the kinder explanation first when somebody serving you is short with you. Be easy to deal with, say thanks and move on without making it a thing. If you work a long shift yourself, notice the hour your own patience goes and warn the people around you.