California Code, Public Utilities Code - PUC § 328.2

The commission shall require each gas corporation to provide bundled basic gas service to all core customers in its service territory unless the customer chooses or contracts to have natural gas purchased and supplied by another entity.  A public utility gas corporation shall continue to be the exclusive provider of revenue cycle services to all customers in its service territory, except that an entity purchasing and supplying natural gas under the commission's existing core aggregation program may perform billing and collection services for its customers under the same terms as currently authorized by the commission, and except that a supplier of natural gas to noncore customers may perform billing and collection for natural gas supply for its customers.  The gas corporation shall continue to calculate its charges for services provided by that corporation.  If the commission establishes credits to be provided by the gas corporation to core aggregation or noncore customers who obtain billing or collection services from entities other than the gas corporation, the credit shall be equal to the billing and collection services costs actually avoided by the gas corporation.  The commission shall require the distribution rate to continue to include after-meter services.