Invoice harassment

MD Inc provided services to my elderly mother in the emergency room of Vincent Pallotti Hospital in February 2019. They submitted invoices that were forwarded to Discovery medical aid who settled them in full except for an item with a claimed value R421.90 which Discovery determined was not due. In Discovery's words "We have not paid an amount on this line as the cost for this item is already included in our agreement with the provider. You do not have to pay this amount." MD Inc have continued to harass us for this amount and refuse to respond meaningfully to any verbal or written communications with them.