Except where otherwise specifically provided, any notice, demand or request required or authorized by this Part, by the applicable service tariff or by the accepted application for electric service shall be deemed properly given if mailed by certified mail, postage prepaid to office of secretary of authority at the address of authority, and to the official signing for customer at the address of customer shown on the application for electric service or other contract document unless another person has been designated to receive such notice, demand or request. The designation of the person to be notified or the address of such person may be changed at any time by similar notice.