Mont. Code Ann. § 69-12-502

It is unlawful for any Class A motor carrier to charge, demand, receive, or collect any greater or less rate, charge, or fare than that fixed by the commission for the transportation service provided. It is also unlawful for any Class A motor carrier to refund or remit, in any manner or by any device, any portion of the rates, fares, and charges required to be collected under the schedule of the Class A carrier on file with the commission.