Principles of International Commercial Contracts, 1994 - UNIDROIT
Chapter 2 - Formation
Article 2.11 - Modified Acceptance
(1) A reply to an offer which purports to be an acceptance but contains additions, limitations or other modifications is a rejection of the offer and constitutes a counter-offer.
(2) However, a reply to an offer which purports to be an acceptance but contains additional or different terms which do not materially alter the terms of the offer constitutes an acceptance, unless the offeror without undue delay, objects to the discrepancy. If the offeror does not object, the terms of the contract are the terms of the offer with the modifications contained in the acceptance.