Overcharged

I went to studio88 to buy my son a pair of sneaker. There was no price on the shelf so i ask one of the shop assistant and she told me that all the kiddies sneakers on this section is R600. When i get to the till she sneaker scanned R650. So I told the cashier to call the lady that assisted me because i was not agreeing with the price. She then said to me it was on promotion so that is not the price. So requested to pay the price i was told they all refused. I requested for the supervisor but they refused to show me who is their supervisor. Invoice number IN*** Date:2021/12/21