I had a guy come up to check out just a few weeks ago who was pretty unreasonable as well. Upon checking him out of the system and charging his credit card, he decides after I'm finished to complain that he didn't get the AARP discount.

Me: I apologize, sir. If you could just show me your AARP membership card, I would be more than happy to apply the discount and re-credit the appropriate amount to your credit card.

Him: Well, I don't have my fucking card on me.

Me: Well, then I apologize, sir, but I can't just apply the discount without proof of membership.

Him: I am 65-fucking-years old, I was told on the phone that I'd get a 10% discount, and goddammit I'm going to get that fucking discount!

Me: (Anger now reaching critical mass) Again, I am sorry sir, but unless you provide proof of your membership, I can't offer you the discount.

Him: (literally ten minutes of ranting about how he's old, deserves the discount, I should be ashamed of myself, blah, blah all the while making the people behind him wait angrily)

Me: OKAY FINE. (I apply the discount, re-credit his card and immediately turn my attention to the next person in line without saying another word)

Him: You know people usually say "Thank You" to their guests.

Me