Article 27 Montreal Convention
Article 27 of the Montreal Convention (Convention For the Unification of Certain Rules for International Carriage by Air) is titled Freedom to Contract. It is under Chapter III (Liability of the Carrier and Extent of Compensation for Damage).
Nothing contained in this Convention shall prevent the carrier from refusing to enter into any contract of carriage, from waiving any defences available under the Convention, or from laying down conditions which do not conflict with the provisions of this Convention.
See also: Article 26 Montreal Convention