This chapter shall not be construed to cover or include aircraft used exclusively in transporting or handling United States mail, or aircraft while used exclusively in interstate commerce; provided, however, that any common carrier of passengers by airplane holding proper authority for and operating in interstate commerce on January 1, 1970, in the transportation of passengers between any licensed airport adjacent to or within three miles of the Virginia State Line on the one hand and any other licensed airport in Virginia on the other, shall, without further proceedings, be issued a certificate by the Commission authorizing such operations as a common carrier of passengers by aircraft in intrastate commerce if application is made to the Commission within 120 days after this section takes effect. Pending the determination of any such application, the continuance of such operation without a certificate shall be lawful.