Cal. Public Utilities Code § 2100

Whenever the commission, after a hearing, finds that any common carrier, other than a railroad corporation, has charged, collected, or received a lesser compensation for the transportation of persons or property, or for any service in connection therewith, than the applicable rates, fares, and charges specified in its tariff schedules filed and in effect at the time, or that the carrier has refunded or remitted in any manner or by any device any portion of the rates, fares, or charges so specified without an order of the commission so authorizing, or charged, collected, or received a lesser compensation than that established by the commission under subdivision (b) of Section 493, the commission shall require the carrier to collect the undercharges involved and may impose upon the carrier a fine equal to the amount of the undercharges. These fines shall be deposited in the General Fund. The remedy and penalty provided by this section shall be cumulative and shall not be a bar to or affect any other remedy or penalty provided for in this part, or to the exercise by the commission of its power to punish for contempt.