Principles of International Commercial Contracts, 1994 - UNIDROIT
Chapter 2 - Formation
Article 2.9 - Late Acceptance. Delay in Transmission
(1) A late acceptance is nevertheless effective as an acceptance if without undue delay the offeror so informs the offeree or gives notice to that effect.
(2) If a letter or other writing containing a late acceptance shows that it has been sent in such circumstances that if its transmission had been normal it would have reached the offeror in due time, the late acceptance is effective as an acceptance, unless without undue delay, the offeror informs the offeree that it considers the offer as having lapsed.