If any such municipal or cooperative system elects to impose a charge for late payment for electric service, it shall show on each bill the amount currently billed, the total amount due and any arrears and late payment charge, each separately stated, the amount of the monthly percentage charge for late payment and when such charge becomes applicable. In no event shall any late payment charge become applicable less than 20 days after the date payment was due. The date payment is due may not occur before personal service of the bill or three days after the mailing of the bill.