Job done incorrect

Hi I have been a client with this pharmacy for more than 20 years. I got medicine in November by them for my back and was told that a payment of 18 cents was payable. A week and a few days later I got a WhatsApp stating that I owe them R283.03. I paid that amount today. I mean how can I be told one amount one day and then later and bigger amount. Somebody is not doing their job correctly and now I have to pay for someone else's mistake. I don't think it's fair. If they told me on the day that I would have to pay the R283.03, I would have never taken the medication.