Bad service

I went to Shoprite Theku plaza to buy my grocery.I paid wth my debit card there were promotions and I made sure that I buy some items on promotion but when I left the till the cashier didn't give me the slip with my free airtime on top of dat where I look at my slip at home the cashier has charged 3maize meal instead of two.I went back today the lady who assisted me said they can't help me i was supposed to comebck yesterday.