Subject to subsection (b) of this section, a person publishing a newspaper or periodical in the State may not charge a candidate for State or local public office a rate for political advertising that exceeds the local rate regularly charged for commercial advertising by the person publishing that newspaper or periodical. If a candidate uses an advertising or press agency to place a political advertisement, the person publishing the newspaper or periodical may charge the national rate regularly charged by that newspaper or periodical for commercial advertising.