Cal. Pub. Util. Code § 532

Except as in this article otherwise provided, no public utility shall charge, or receive a different compensation for any product or commodity furnished or to be furnished, or for any service rendered or to be rendered, than the rates, tolls, rentals, and charges applicable thereto as specified in its schedules on file and in effect at the time, nor shall any public utility engaged in furnishing or rendering more than one product, commodity, or service, charge, demand, collect, or receive a different compensation for the collective, combined, or contemporaneous furnishing or rendition of two or more of such products, commodities, or services, than the aggregate of the rates, tolls, rentals, or charges specified in its schedules on file and in effect at the time, applicable to each such product, commodity, or service when separately furnished or rendered, nor shall any such public utility refund or remit, directly or indirectly, in any manner or by any device, any portion of the rates, tolls, rentals, and charges so specified, nor extend to any corporation or person any form of contract or agreement or any rule or regulation or any facility or privilege except such as are regularly and uniformly extended to all corporations and persons. The commission may by rule or order establish such exceptions from the operation of this prohibition as it may consider just and reasonable as to each public utility.