Overcharged

I made a purchase of a product and only realised after paying that i was overcharged for the product i made the cashier aware of this she called the manager explained to him the issue he proceeded to tell me that I should have looked at the barcodes i told him its not my responsibility to check if the price they put on a product barcodes are the same contacted his head office he lies and says the product was misplaced of wich it wasnt The product was neatly packed with that pricing