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UCC 2-207(1) is subject to a proviso. If a definite and seasonable expression of acceptance expressly conditions acceptance on the offeror's assent to additional or different terms contained therein, the parties' differing forms do not result in a contract unless the offeror assents to the additional terms. See J. White & R. Summers, § 1-2 at 32-33. If the offeror assents, the parties have a contract and the additional terms are a part of that contract. If, however, the offeror does not assent, but the parties proceed with the transaction as if they have a contract, their performance results in formation of a contract. ...