New York Laws
Part 2 - Form, Formation and Readjustment of Contract
2-202 - Final Written Expression: Parol or Extrinsic Evidence.

(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.