Cal. PUC § 528

Nothing in this part shall prevent the interchange of free or reduced rate transportation for passengers or express matter between a common carrier and a corporation engaged in the carriage of persons or property by motor vehicle, or the officers, agents, employees, attorneys, physicians and surgeons of such common carrier or corporation and their families when the corporation is engaged in the carriage by motor vehicle of persons or property as a connecting carrier, or in the carriage by motor vehicle of persons or property over a portion of a route between the point of departure or shipment and arrival or delivery, part of which route is covered or traveled by a common carrier, whether the carriage by motor vehicle is an initial, an intermediate, or a final portion of the entire route between the point of departure or shipment and arrival or delivery.

This section does not authorize the interchange of free or reduced rate transportation for express matter which is not for the personal use of the person to or for whom it is granted, or of his family.