Wis. Stat. § 402.202: Final written expression: parol or extrinsic evidence.

Terms with respect to which the confirmatory memoranda of the parties agree or which are otherwise set forth in a writing 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:

(1) By course of dealing or usage of trade (s. 401.303) or by course of performance (s. 402.208);

(2) By evidence of consistent additional terms unless the court finds the writing to have been intended also as a complete and exclusive statement of the terms of the agreement.