I do understand your negative reaction to the clerk. She certainly should not have been rude to you or anyone.
I remember once when I didn't stop at a "STOP HERE ON RED" sign just before a crosswalk and the oncoming, turning onto this street, bus driver cursed me and motioned nasty things with his hands at me.
What he didn't know was I had just been given news that I possibly had cancer and yes, even though I was in the wrong, I didn't need the anger he showed.
We tend to forget that people may be having a bad day and I have found that if I approach them and ask "Is something wrong? I know you would never have spoken that way if everything was okay."(in a caring tone, not a nasty one!) It sometimes brings out a better side of them. It sometimes brings out an apology. And if they still act huffy, puffy and downright nasty, THEN I complain to the management. Let's at least give someone the benefit of the doubt.