Cal. Rev. & Tax. Code § 41075
The department shall give to the service supplier, seller, or service user written notice of its determination. The notice shall be placed in a sealed envelope with postage paid addressed to the service supplier, seller, or service user at the service supplier, seller, or service user's address as it appears in the records of the department. The giving of notice shall be deemed complete at the time of the deposit of the notice in the United States post office or facility regularly maintained or provided by the United States Postal Service, without extension of time for any reason. In lieu of mailing, a notice may be served personally by delivering to the person to be served and service shall be deemed complete at the time of such delivery. Personal service to a corporation may be made by delivery of a notice to any person designated in the Code of Civil Procedure to be served for the corporation with summons and complaint in a civil action.
Ca. Rev. and Tax. Code § 41075