Mo. Rev. Stat. § 115.285: Secretary of state may prescribe regulations as to printing absentee ballot and mailing envelopes, no cost to voter.

The secretary of state may prescribe uniform regulations with respect to the printing of ballot envelopes and mailing envelopes, which shall comply with standards established by federal law or postal regulations. Mailing envelopes for use in returning ballots shall be printed with business reply permits so that any ballot returned by mail does not require postage. All fees and costs for establishing and maintaining the business reply and postage-free mail for all ballots cast shall be paid by the secretary of state through state appropriations.