turquise towel trouble

I had the weirdest conversation with this shop assistant the other day.

I was looking to buy some new bath towels (well, my towels are nearly 10 years old....), and found these lovely deep turquoise ones. The price ticket on the shelf said 80.-/each, but on a promotion table it said 3 bath towels - 270.-

I walked up to the till and asked the shop assistant which price the towels were. She said "80.- each, but you get three for 270.-". I explained that 270.- divided in three was in fact 90.-, and so it was obviously cheaper to buy one towel three times?

She looked at me as if I was a lunatic. "No", she said firmly, "they're on special offer right now, so you get three for 270.-!" I explained again. I wanted three towels, but I wanted to pay 80.-/each, and therefor would buy one at the time to get the cheaper price. 

The woman behind me in the queue started giggling. This made the shop assistant even more rabid, and she started to explain that sometime the shop has certain things on special offer, which indeed these towels were this week, and therefor they were cheaper if I bought three for the price of 270.-. She finished the whole lecture by saying "now do you get it?!"

I couldn't help but carry on arguing with her. I said that No, I most certainly didn't get it, "as 80 x 3 = 240, and that the last time I checked 240.- were cheaper than 270.-. Now, did she get that?!"

And there it was. I actually saw the penny drop in her head. She went bright red, thought for a split second  and said loud and clear for the whole queue to hear "yes, but the towels for 240.- are smaller than the ones for 270.-". And I thought I'd spare her the embarrassment to confront her that there were no different sized ones, in fact. So I said nothing, smiled and paid 240.- for my lovely new towels.