Mich. Comp. Laws § 440.2205: Firm offers.

Sec. 2205.

An offer by a merchant to buy or sell goods in a signed writing which by its terms gives assurance that it will be held open is not revocable, for lack of consideration, during the time stated or if no time is stated for a reasonable time, but in no event may such period of irrevocability exceed 3 months; but any such term of assurance on a form supplied by the offeree must be separately signed by the offeror.