In the event the bill is not paid by the due date, a 10% penalty will be added to the net amount of the bill, and both shall become immediately due and payable. All payments by mail must be postmarked on or before the due date to prevent imposition of the 10% penalty. When the due date falls on a legal holiday, Saturday or Sunday, the net amount will be accepted on the first business day following. Failure to receive the bill shall not excuse a late payment penalty unless it can be shown from the billing record that the bill was not sent.