(a) In the performance of his duties and in relation to any investigation or inquiry which the commissioner is authorized to conduct, the commissioner or any agent designated by him in writing may:

(1) Administer oaths and take affidavits;

(2) Conduct hearings;

(3) Examine witnesses under oath; and

(4) Subpoena the attendance of witnesses and require the production of books, papers, records, and documents and, subject to the rights of the taxpayer as to rights of privacy guaranteed to the taxpayer by the Constitution and laws of this state, may examine such items and the books, records, inventories, or business of any taxpayer or of any fiduciary, bailee, or other person having knowledge of the tax liability of any taxpayer or knowledge pertinent to the investigation or inquiry. The subpoena may be served by the commissioner or the commissioner's authorized representative to such person at the person's last known address by registered or certified mail or statutory overnight delivery, return receipt requested. If such person refuses to accept service of a subpoena by registered or certified mail or statutory overnight delivery, the subpoena shall be served by the commissioner or the commissioner's authorized representative under any other method of lawful service, and the person shall be personally liable to the commissioner for a sum equal to the actual costs incurred to serve the subpoena. This liability shall be paid upon notice and demand by the commissioner or the commissioner's delegate and shall be assessed and collected in the same manner as other taxes administered by the commissioner.

(b) The powers conferred pursuant to subsection (a) of this Code section shall be exercised with due regard to the rights of the citizen and, when invoked, subject to the approval of the superior courts of this state.

(c) The commissioner, pursuant to his duties in relation to the collection of state ad valorem taxes, shall investigate settlements by tax collectors or tax commissioners and take appropriate action to collect any revenue due the state which has not been collected or, having been collected, has not been paid to the commissioner. No official or person may employ or commission any person to collect any of such taxes on a commission basis.