(810 ILCS 5/2-202)(from Ch. 26, par. 2-202) Sec. 2-202. 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 (a) by course of performance, course of dealing, or
usage of trade (Section 1-303); and
(b) 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.