Idaho Code § 61-310: Only schedule rates to be charged.

No common carrier except as in this act otherwise provided shall charge, demand, collect or receive a greater or less or different compensation for the transportation of persons or property, or for any service in connection therewith than the rates, fares and charges applicable to such transportation as specified in its schedules filed and in effect at the time; nor shall any such carrier refund or remit in any manner or by any device any portion of the rates, fares or charges so specified except upon order of the commission as hereinafter provided, nor extend to any corporation or person any privilege or facility in the transportation of passengers or property except such as are regularly and uniformly extended to all corporations and persons.