The Department of Transportation, upon written application of any person authorized by the United States government to carry mail by motor vehicle, or of any person desiring to carry passengers for hire to and from any rural section where there is no other agency of public transportation of passengers, may authorize such applicant to transport passengers for hire in such motor vehicle over a prescribed route if, in the opinion of the department, public convenience and necessity require the same. The department shall also determine the registration fee, if any, to be charged such applicant and shall forward to the Commissioner of Motor Vehicles a certified copy of its findings concerning the requirements of public convenience and necessity and the registration fee, and, thereupon, said commissioner may register such vehicle for such service. The department may, at any time, amend or revoke any such authorization. Any such authorization issued by the Division of Public Utility Control within the Department of Business Regulation prior to October 1, 1979, shall remain valid unless revoked by the Department of Transportation. Said department may make rules, regulations and orders relating to such passenger service and fixing rates and schedules therefor, provided such rules, regulations and orders shall not be inconsistent with federal regulations pertaining to carriers of United States mail.