(a) All orders, notices, interpretations or other documents required to be served under this part shall be served personally or by registered or certified mail or by regular United States mail (only when service is effected by the DOE or a State Office), except as otherwise provided.(b) Service upon a person's duly authorized representative shall constitute service upon that person.(c) Service by registered or certified mail is complete upon mailing. Official United States Postal Service receipts from such registered or certified mailing shall constitute prima facie evidence of service.