Mo. Rev. Stat. § 140.850

For all taxes administered by the department of revenue, the director may enter into contracts with private attorneys or professional collection agencies for the collection of delinquent taxes owed by residents or nonresidents of the state of Missouri; except that, any contract let pursuant to this section shall be awarded in the manner prescribed by chapter 34 and shall be subject to appropriation made therefor. Any contract entered into pursuant to this section shall not provide for a collection fee in excess of twenty-five percent of the amount collected.