Cal. Pub. Util. Code § 737.3

(a)
1. (1) A highway carrier, as defined by subdivision (c), a freight forwarder, a party representing a carrier or freight forwarder, or an assignee of a carrier or freight forwarder shall not, based on a filed tariff or a filed contract, collect or attempt to collect any additional charge in excess of the charge originally billed by the carrier or freight forwarder for transportation service previously provided subject to the jurisdiction of the commission, except where there are mistakes in billing that are acknowledged by both parties or that are the result of intentional misrepresentation by the shipper.
2. (2) Similarly, the person or entity against whom a claim has been made under the circumstances described in paragraph (1) shall not be liable for additional amounts based on a filed tariff or a filed contract, except where there are mistakes in billing acknowledged by both parties or that are the result of intentional misrepresentation by the shipper.
(b) For the purposes of this section, the term “claimant” shall mean the carrier or freight forwarder, or its assignee or representative making a claim for the collection of rates and charges in addition to those originally billed and collected for the transportation.
(c) “Highway carrier” or “carrier” means every corporation or person, their lessees, trustees, receivers or trustees appointed by any court whatsoever, engaged in transportation of property for compensation or hire as a business over any public highway in this state by means of a motor vehicle, except that “highway carrier” does not include:
1. (1) Any farmer resident of this state who occasionally transports from the place of production to a warehouse, regular market, place of storage, or place of shipment the farm products of neighboring farmers in exchange for like services or for a cash consideration or farm products for compensation.
2. (2) Persons or corporations hauling their own property.
3. (3) Any farmer operating a motor vehicle used exclusively in the transportation of his or her livestock and agricultural commodities or in the transportation of supplies to his or her farm.
4. (4) Any nonprofit agricultural cooperative association organized and acting within the scope of its powers under Chapter 1 (commencing with Section 54001) of Division 20 of the Agricultural Code to the extent only that it is engaged in transporting its own property or the property of its members.
5. (5) Any person exclusively transporting United States mail pursuant to a contract with the United States government.
6. (6) Any integrated intermodal small package carrier which is registered subject to Chapter 2.7 (commencing with Section 4120).
7. (7) Any household goods carrier, as defined in Section 5109.
(d) For purposes of this section, “mistakes in billing” include, but are not limited to, matters such as clerical errors, billing for transportation of a different commodity than the commodity actually shipped, and billing for transportation of a smaller amount of the commodity than the amount actually shipped.
(e) This section shall apply to all claims arising from transportation performed (in whole or in part) before January 1, 1995, including all lawsuits or claims pending on the effective date of this section.
(f) If any claim that qualifies under this section was settled by mutual agreement of the parties to the claim, or resolved by a final adjudication of a federal or state court, before the effective date of this section, the settlement or adjudication shall be treated as binding, enforceable, and not contrary to law, unless the settlement was agreed to as a result of fraud or coercion.
(g) If the claimant has filed, on or before the effective date of this section, a suit for the collection of additional freight charges, the claimant shall notify the person, or entity, from whom additional freight charges are sought of the provisions of this section within 30 days of the effective date of this section.
(h) If, on or before the effective date of this section, the claimant has demanded the payment of additional freight charges and has not filed a suit for the collection of additional freight charges, the claimant shall notify the person, or entity, from whom additional freight charges are sought of the provisions of this section within 30 days of the effective date of this section.