People are rude assholes to cashiers all day long. They really appreciate a little bit of kindness. If you're hung up on thinking you're retarded for saying "thank you" when you get you change, then change your thinking so that you are thanking them for doing their job and helping you out. Add a smile and follow it up with "have a good day". When my kids were small, we went to Wisconsin Dells and got some McDicks drive through after a day in a waterpark. When placing my order, I said my usual "Hi, may I please have a (insert food items)". When I pulled up to the window to pay, it was am elderly lady there, holding back tears, she thanked me for my manners and said "all I get all day are rude, snappy people who only say "give me a...... I need a.....". Manners don't cost you anything to share, and you never know when it will make someone's day.