If you have a problem with a sales clerk, don't tell them.  Tell a manger.  You don't know that clerk or their situation.  Maybe their dog died last night; you don't know.  But I guarantee that if you tell a clerk--a human being--how awful they are to their face, it will 100% ruin their day.  It will be in the back of their mind with every customer they help after you.  They might be driven to tears.  They might be driven to quit.  How dare you hold that much control over another human being.

No on one this planet it perfect.  I bet there is a time in your life that you were (accidentally, intentionally, it doesn't matter) rude to someone.  Or that your tone of voice or choice of words offended someone.  Or that you maybe forgot a piece of information that someone else thought was very important.  I implore you to remember that before you insult someone or snap at someone.

Better yet, let's not limit this to sales clerks.  Just be nice to people in general.  You'd be surprised at how much easier that makes your life.

PS: I was blocked from entering the freeway this morning by a car with a COEXIST bumper sticker.  Awesome.