arguing with the customer

I had an unfortunate incident yesterday due to the signage for the Sorbet 3 for 2 offer being vague. The offer states \buy 3 sorbet products and get the lowest priced one free". I therefore decided to treat myself to 3 bottles of nail polish. I also bought some other goods at the same time and decided to check that the Sorbet offer had gone through before the sale was rung up. It hadn't and the cashier commented that it may have been because the 3 polishes were the same price so the system couldn't process the offer. She then called the Manageress over who went to get the signage off the shelf and came back and pointed to the photo on it saying \""it has to be 3 different Sorbet products\"". As much as I tried to explain that the wording didn't state that she kept talking over me"