How you treat the cashier is the most honest choice anyone sees you make all day.

Being decent to somebody who cannot affect you is the clearest evidence anybody gets about what you are like, precisely because nothing is being bought with it. People register it faster than they register achievements, and they keep the conclusion. The person on the till is also having a day, largely made of people who looked straight past them.

Look up, use the name on the badge, and say thank you as though to a person rather than to a process. When something has gone wrong, be the customer who clearly is not blaming whoever cannot fix it. Four seconds, every time.