Article 21 CISG
(1) A late acceptance is nevertheless effective as an acceptance if without delay the offeror orally so informs the offeree or dispatches a 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 delay, the offeror orally informs the offeree that he considers his offer as having lapsed or dispatches a notice to that effect.