Cal. Rev. & Tax. Code § 6715

In any action relating to the use tax brought under this part process may be served according to the Code of Civil Procedure and the Civil Code of this State or may be served upon any agent or clerk in this State employed by any retailer in a place of business maintained by the retailer in this State. In the latter case a copy of the process shall forthwith be sent by registered mail to the retailer at his principal or home office.