Cashier Justifies her Mistake by Forcing Pensioner's to Pay

We are pensioners, and we went to cashier Nokuthula to pay. She entered R 358.95 on the terminal, which I paid with my card. I asked for my Invoice, and then she saw "SHE MADE A MISTAKE - Entered the incorrect amount on the terminal? In a cheeky manner she demanded that I pay R20 cash which was the short fall, from her mistake. I asked her why, but she got more arrogant, and would not give me my Invoice, till I had Paid. She said that "these mistakes happen!!!!! She did not even say "Sorry" or "Apologise" - Instead she ignored us. Shame that we as Pensioner's are treated in this manner?