Medi Rite does not honor consumer law

Medi+Rite has *********** ***** working for them. On 28/01/2015 my mother went to Medi+Rite in Shoprite Gezina, Pretoria. I gave her R500.00 for the Spesadex.Co Eye drops she was to get which were still on prescription. The normal people that worked there were not working that day and it seems that whoever the replacements were had no idea what they were doing. She was told the amount for the items was R158.45 which is what she paid. She phoned me and even I was shocked since I knew that was the price of one bottle. She went back and spoke to the White female manager who confirmed it was correct. So she left it and brought me back the rest change. Three weeks later until present Medi+Rite Gezina has been threatening us with lawyers for their own mistakes and still want us to pay another R360.00, I have been in retail for over 25 years and even I know consumer law. The price you see is the price you pay and if mistakes were made by workers that is their problem. I did not have a problem originally paying it on the above date but when they started to become rude with threats I decided not to and I still refuse to. Threats not welcome.