Ask questions instead of fighting back

How many times have we been confronted with people in customer service who confound us when they can't do something for us?  I had one such experience in convenient store of all places this morning.  No shock to people who know me well, but I had earned a "free soft drink" after purchasing 9 diet cokes in the last couple of weeks.  I told the gentleman at the counter, "I think this is my free one."  He rang up the purchase and told me I owed $1.55.  "Ummm...I'm confused....I thought this was my free drink." He looked at me slowly and then said, "Maybe, but I won't know that until I put it in the system after you pay."  So, I tried desperately to understand, "So are you saying I have to pay you first and then you will give me my money back?  That doesn't make a lot of sense, does it?"  He looked at me slowly again then said, "Let me see here" and then pushed a button and said, "It's free".  Now, at this point, I have a couple of options, one being to say "See? I told you!" Instead I simply asked, "Is there something I should say differently the next time to avoid confusion?" He smiled and said, "Just tell me you want to redeem your free one."
At the end of the day, do I want to be right or do I want to make it alright?  I don't always choose this route, by the way and I DESPERATELY wanted to say "That's what I told you in the first place!!" But the difference between what we want to say and what we choose to say is often a huge deal. 
Be kind to others----it will pay off in dividends.