Missouri Revised Statutes Title XXVI. Trade and Commerce § 415.417. Late fee assessed, when, amount--recovery of expenses, when

1. For the purposes of this section, “late fee” means a fee or charge assessed by an operator for an occupant's failure to pay rent when due. A late fee is not interest on a debt, nor is a late fee a reasonable expense which the operator may incur in the course of collecting unpaid rent in enforcing his or her lien rights pursuant to sections 415.400 to 415.425, or enforcing any other remedy provided by statute or contract.

2. Any late fee charged by the operator shall be stated in the rental agreement. No late fee shall be collected unless it is written in the rental agreement or an addendum to such agreement.

3. An operator may impose a reasonable late fee for each month an occupant does not pay rent when due.

4. A late fee of twenty dollars or twenty percent of the monthly rental amount, whichever is greater, for each late rental payment shall be deemed reasonable, and shall not constitute a penalty.

5. An operator may set a late fee other than that permitted in subsection 4 of this section if such fee is reasonable. The operator shall have the burden of proof that a higher late fee is reasonable.

6. The operator may recover all reasonable rent collection and lien enforcement expenses from the occupant in addition to any late fees incurred.