Bad service and wrong prices = Profits

It is happening every week- the price on the shelves are not the same as when you pay. I have been monitoring it for a month and have experienced it every week at ShopRite Florida. And when I complain it's so convenient the Till supervisors are nowhere to be found.....eg: I bought a set of Combs, shelve price was 29.99 but I was charged 39.99......etc. Is this how ShopRite makes their profits? The service of cashier's ....not sure what criteria is used to hire their staff cause it's as if they are doing us a favor when we stand in ques to pay for our goods.