What does "additional % off" mean to you?

Today was pretty awesome. It was my mom's birthday - a big milestone birthday! I was teasing her all day that she was turning the big "21" because 50 is the new 21. Fun times! I took her to Starbucks for "breakfast" then we ran some errands, and we met my brother (in town from Montana!) at the Alehouse downtown where we had lunch for mom's birthday. She got a free beer and I got some yummy nachos!

Meanwhile, while mom was at one of her appointments today, I went to Sears. I go to Sears all the time. They have some really good sales regularly, so I like to hit the sale racks in the baby clothes, shoes, etc. Today, I was looking at tennis shoes for Katie. They were Avia brand, marked down from $19.99 to $7.99. Good deal, right? Well even cooler, there was a sign by the door that read "take an additional 30% off women's and children's clearance shoes and boots". SCORE! I was thinking great, now I can get 2 pair for about $10 for Katie!

So I go to the register and the guy said he'd have to ask his manager. The manager laughs at me and says no, they're as low as they're going to go. He muttered something about them being super cheap already, what more did I want?

I told the cashier I'd put the shoes back. As I was leaving the store, I noticed the manager a couple rows of shoes away. I grabbed the sign that said "take an additional 30% off women's and children's clearance shoes and boots" and handed it to him. I advised him "You may want to take this down then because it's misleading". He ARGUED with me! He said that the 30% is already taken and it's included in the ticketed price. I informed him that it shouldn't say "additional" then. And he said "Well, I can't control how you interpret the sign", and promptly hung it back up.

Maybe I got a bit carried away, but I was pretty ticked the guy laughed at me and practically called me an idiot without saying the word. Not only that, what ever happened to "the customer is always right"? I know I'm not always right, but the guy was just plain rude even before I started trying to push my point.