Nothing in this title shall be construed to prevent any of the parties named in the first article hereof, publishers, editors or proprietors of newspapers or magazines, from making an exchange of mileage for advertising space in such newspaper or magazine, provided the contract between the railway companies and publishers, editors or proprietors of such newspapers or magazines shall be at the same rate as is charged the public generally for like service, providing that such contract shall be in writing and shall not be operative until approved by the Railroad Commission of this State, and filed in the office of such Commission, subject at all times to a reasonable public inspection.