The mailbox rule, which is the default rule under contract law for determining the time at which an offer is accepted, states that an offer is considered accepted at the time that the acceptance is mailed. So, if the offeree can argue that a properly posted acceptance should be a reasonable method of acceptance, then the mailbox rule is technically upheld. In the United States, the majority rule is that the mailbox rule does not apply to option contracts. By default, an option contract is accepted when the offeror receives the acceptance, not when the offeree mails it.
Regarding the mail box rule under common law, Wiley 2000 CPA Exam Review(P66 h.) said that if acceptance is made by SAME method used by offeror to communicate the offer. The UCC adopts the common law mail box rule and the Convention adopts a receipt rule. Under the UCC, acceptance of an offer is effective when mailed; The UCC acknowledges the mailbox rule, but relaxes the common law’s treatment by requiring that acceptance need only be made in a reasonable manner by a reasonable medium.
The general rule is that acceptances are effective on dispatch (when they are mailed). Even under the UCC a conditional acceptance is a rejection/counter-offer. Offeror, as master of the offer, can contract out of the mailbox rule and make the acceptance effective upon receipt. The UCC will cover any sale of moveable goods which includes commodities like rice, alcohol, cars, etc.