Section 7. (a) Every contract carrier by motor vehicle, other than contract carriers by armored motor vehicles of bullion, currency, securities, negotiable and non-negotiable documents, jewels, and other precious and very valuable articles, shall file with the department a copy of every contract for the transportation of property by motor vehicle in effect in connection with its operations, which shall be in writing and shall be executed by all parties thereto. The period of time during which such contract shall be in force, the charges for transportation and accessorial service, if any, and the duties and obligations of all parties thereto, shall be specifically stated therein, and said copies shall be kept on file in the department, and shall be open to public inspection. The department may also in its discretion require all contract carriers to file schedules of their minimum charges, each of which schedules shall contain a list of all parties with whom contracts are or may from time to time be in effect, and a statement of the charge for each service performed thereunder, without designation of the party for whom the same are being or are to be performed. Such schedules shall be kept on file in the department and shall be open to public inspection. Any change in such minimum charges shall become effective not less than thirty days after the filing of such change with the department; provided, that the department may in its discretion and for good cause shown allow such change to become effective within a lesser period.

(b) No such carrier shall demand, charge or collect compensation for such transportation different from that provided for in his written contract, or less than that contained in the minimum schedule of charges, if any, filed therewith, or prescribed by the department from time to time, and no such carrier, by the furnishing of any special service, facility or privilege, or by any other device whatsoever, shall in effect charge, accept or receive for any transportation or service less than the minimum charge, if any, established therefor under this section.

(c) Whenever, after hearing, upon a complaint or upon its own initiative, the department finds that any charge of any contract carrier by motor vehicle or any rule, regulation or practice of any such carrier affecting such charge or the value of the services thereunder for the transportation of property upon ways, contravenes the policy declared in section one or causes an undue or unreasonable advantage or preference to any such carrier in competition with any other motor carrier, the department, giving due consideration to the cost of services rendered by such carrier, may prescribe such charge, rule, regulation or practice as in its judgment may be necessary or desirable in the public interest. The charges of said contract carriers by motor vehicle for transportation shall in general be no less than those of common carriers by motor vehicle for the same or similar service.