Art. 19 CISG
(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 orally to the discrepancy or dispatches a notice to that effect. If he does not so object, the terms of the contract are the terms of the offer with the modifications contained in the acceptance.
(3) Additional or different terms relating, among other things, to the price, payment, quality and quantity of the goods, place and time of delivery, extent of one party's liability to the other or the settlement of disputes are considered to alter the terms of the offer materially.
The Secretariat Commentary on Art. 17 of the 1978 New York Draft, predecessor to Art. 19 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.
CISG-AC Opinion regarding Art. 19 CISG
CISG-AC Opinion No. 13 ‘Inclusion of Standard Terms under the CISG’ (Rapporteur: Sieg Eiselen), adopted by the CISG Advisory Council following its 17th meeting in Philadelphia (USA) on 20 January 2013
Overview of case law on Art. 19 CISG: UNCITRAL Digest
The UNCITRAL Digest on the CISG can be used as a starting point to discover the relevant case law on this article.