In its opinion, the ICSG Advisory Committee noted that the CISG does not include the Parol rule of evidence.  Article 11 of the ICSG stipulates that „a sales contract must not be entered into or proven in writing and is not subject to any other formality obligation. It can be proven by any means, including by witnesses.  The commentary of the editorial conference explained that this provision was included because an overwhelming number of „international sales contracts are entered into by modern means of communication that may not always include the letter.“  In an increasingly globalized world, where trade and communications have become largely electronic and subject to technological progress, such as the advent of e-commerce, it is conceivable that the letter alone cannot be considered an integral or precise expression of the intentions of the parties. While you can work out an agreement yourself, it`s best for you to consult a professional experienced in developing similar agreements and give you an overview of what you need to pay attention to and what you need to avoid. There are several factors that we often forget when developing an agreement and it is important that all relevant details are taken into account so that there is little or no ambiguity in the agreement, especially in the event of a dispute. If an oral contract does not interfere with one or more elements of a valid contract, it is likely that a court will declare the agreement inconclusive and unenforceable. Many states have written provisions for certain treaties that believe that oral agreements are insufficient. Among the main justifications for the introduction of the rule are the legal uncertainty that arises from the memory of a person on which oral agreements and statements are based, and the risk of forgery, fraud or perjury.  Other justifications are put forward in two approaches.
The „approval process“ recognizes that the parties intend to make the letter a manifestation of their final agreement superior to all previous agreements.  The „quality“ approach recognizes that a carefully crafted written agreement to reflect the intentions of the parties should be more taken into account and more important than any other agreement or agreement between the parties.  The objective is to ensure that weaker evidence cannot alter or corrupt the strongest written evidence.  It is not necessary to write any of these points.