13 GCA § 2202: 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 dealing or usage of trade (Section 1205) or by course of performance (Section 2208); and

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