New York Laws
Part 2 - Form, Formation and Readjustment of Contract
2-207 - Additional Terms in Acceptance or Confirmation.

(1) A definite and seasonable expression of acceptance or a written
confirmation which is sent within a reasonable time operates as an
acceptance even though it states terms additional to or different from
those offered or agreed upon, unless acceptance is expressly made
conditional on assent to the additional or different terms.
(2) The additional terms are to be construed as proposals for addition
to the contract. Between merchants such terms become part of the
contract unless:
(a) the offer expressly limits acceptance to the terms of the
offer;
(b) they materially alter it; or
(c) notification of objection to them has already been given or
is given within a reasonable time after notice of them is
received.
(3) Conduct by both parties which recognizes the existence of a
contract is sufficient to establish a contract for sale although the
writings of the parties do not otherwise establish a contract. In such
case the terms of the particular contract consist of those terms on
which the writings of the parties agree, together with any supplementary
terms incorporated under any other provisions of this Act.