Mo. Rev. Stat. § 482.350
Personal service is not required for the service of any summons issuing out of a small claims court, unless specifically requested by the plaintiff. The service may be made by mailing a copy of the summons and the complaint to the defendant at his last known address by certified mail, return receipt requested, delivery restricted to the addressee. The envelope and the return receipt shall be stamped with the docket number of the case. The receipt for certified mail shall state the name and address of the addressee and the date of mailing and shall be attached to the original summons. The return receipt, when signed by the addressee and when returned to the clerk, shall be attached to the original summons, and, if it shows delivery at least ten days before the day for appearance, shall constitute proof of service. The clerk shall note the fact of service in a permanent record.
§ 482.350, RSMo