Charging interest wrongfully

Ever since I had a Telkom account I paid it on the date indicated by Telkom on the account as the due date. My payment was always recorded by them on that exact date but since September 2017 I am being charged interest on late payment. This happened now for two months in a row, Sept and Oct 2017. All of a sudden my payment on due date is now recorded by Telkom only the following day and I have to pay interest. I know it is an insignificant amount but it is the principle that counts: if Telkom charge one million customers R1 interest wrongfully, they score an extra R1 000, 000 in only one month!!!