Cal. COM § 2202

Terms with respect to which the confirmatory memoranda of the parties agree or which are otherwise set forth in a record intended by the parties as a final expression of their agreement with respect to such terms as are included therein may not be contradicted by evidence of any prior agreement or of a contemporaneous oral agreement but may be explained or supplemented: (a) By course of dealing, course of performance, or usage of trade (Section 1303); and (b) By evidence of consistent additional terms unless the court finds the record to have been intended also as a complete and exclusive statement of the terms of the agreement.