The commission shall prescribe minimum rates, fares, and charges for contract carriers. No reduction shall be made in any such charge, either directly or by means of any change in any rule, regulation, or practice affecting such charge or the value of service thereunder, except after ten days' notice of the proposed change filed in the form and manner provided for common carrier rate changes. The notice shall plainly state the change proposed and the time when the change will take effect. No contract carrier shall demand, charge, or collect less compensation for such transportation than the charges to be made by common carriers for the same transportation in accordance with sections 75-118 to 75-155 and 75-301 to 75-322, as affected by any rule, regulation, or practice so filed or as prescribed by the commission. It is unlawful for any such carrier to charge less than the rates, fares, and charges approved by the commission.