The service of a document or notice relating to a proceeding before the Commission under this division shall be sufficient: if made personally through the sheriff’s office in the county in which service may be made or by an adult; or except as provided in subsections (b) and (c) of this section, if mailed by first–class mail, postage prepaid, to the last known address of the person to be served. Service of complaints and accompanying documents shall be sufficient if they are mailed by registered first–class mail, postage prepaid, to the last known address of the person to be served. Service of subpoenas shall be sufficient only if made personally through the sheriff’s office in the county in which service may be made or by an adult.