Over priced

I tried to buy a product there, the shelf price said 2/1, I never check prices, but it was the only thing I was buying, when the cashier told me it was $9.04, I told her must to be a mistake, the shelf said it was 2/1. I talked to the manager in charge she said that the shelf price is incorrect and it is $4 each, I complaint about the error, it wasn't my fault, she should honor the price. She refused, so I asked her to remove the price of the shelf, she refused. (So, many other costumers are paying more of what they expect. Be careful!