Wis. Stat. § 411.202: Final written expression: parol or extrinsic evidence.

Terms with respect to which the confirmatory memoranda of the parties agree or that 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 in the writing may not be contradicted by evidence of an earlier agreement or of a contemporaneous oral agreement but may be explained or supplemented by all of the following:

(1) Course of dealing or usage of trade or by course of performance.

(2) 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.