Mo. Rev. Stat. § 416.535: Proof of service by mailing.

Proof of the mailing or personal delivery of said notification to the nonresident by an adult person not an officer serving same shall be made by affidavit of the party doing said acts. All affidavits of service shall be endorsed upon or attached to the original papers to which they relate and including the returned registry receipt shall be forthwith filed with the court in which the action is filed and pending.