Mo. Rev. Stat. § 365.140

Notwithstanding the provisions of any retail installment contract to the contrary any buyer may prepay in full, whether by payment in cash, extension or renewal, at any time before maturity the debt of any retail installment contract and on so paying the debt shall receive a refund credit thereon for the anticipation of payment. The amount of the refund shall be calculated by the actuarial method. The lender shall retain no more interest than is actually earned whenever a retail installment contract is prepaid. Any insurance rendered unnecessary by reason of prepayment shall be cancelled by the holder and any refund of premiums received by the holder shall be treated in accordance with the provisions of subsection 2 of section 365.080. If a retail installment contract is paid in full, the holder shall provide the buyer proof of payment in full which may be by a letter referencing the contract, which shall include information identifying the contract such as the original loan date, account number or other identifying number or code, or by returning the original contract or a copy thereof that is marked as paid in full by the holder.