Mo. R. Civ. P. 54.16
Service of the summons and petition upon a resident or nonresident defendant of any class referred to in Rule 54.13(b)(1), (2) or (3) may be made by mailing a copy of the summons and petition by first class mail, postage prepaid, to the person to be served, together with two copies of a notice and acknowledgment conforming substantially to Civil Procedure Form 4B or Civil Procedure Form 4C and a return envelope, postage prepaid addressed to the sender. If no acknowledgment of service under this Rule 54.16 is completed and returned to the sender, service of the summons and petition shall be made as otherwise provided by statute or rule. Unless good cause is shown for not doing so, the court shall order the payment of costs of service on the person served if such person does not complete and return within thirty days after mailing, the notice and acknowledgment of receipt of summons.
Mo. R. Civ. P. 54.16