(1) Any sum or amount of money paid to any motor carrier in excess of the rates, fares, or charges established for such service by the commission may be recovered from such carrier by the person or shipper who paid the same in any action brought in the district court of the county in which such payment was made. Any such action must be brought within 2 years from the date of such payment. No contract or agreement, written or otherwise, between such person or shipper and any motor carrier shall be admissible in evidence for the purpose of showing a waiver of the right given by this section.
(2) If upon the trial of such action it shall satisfactorily appear to the court or jury that such overcharge was willfully made, the person or shipper bringing the said action shall be awarded damages in treble the amount of such excess or overcharge, together with the costs and expenses of such action, including a reasonable attorney's fee, to be taxed and collected as other costs in the action.