Mo. Rev. Stat. § 142.908
In the event the tax imposed by this chapter is not precollected and must be paid by the consumer in accordance with section 142.866, the tax is due and payable by the consumer on the last day of each month for the purchases made in the preceding calendar month. The consumer shall file with the director, on forms furnished by the director, a return showing in detail the total purchase price of the motor fuel, the number of gallons purchased or blended, the location of the purchase, the blend stocks and motor fuel components and any other information the director may deem reasonably necessary. With each return, the consumer shall remit to the director the amount of tax shown on the return to be due. Reports timely mailed shall be considered timely filed. If a report is not timely filed, penalties and interest shall be charged from the date the report should have been filed until the report is actually filed and taxes are paid. Penalties shall be five percent of the tax due per month for each month or part of a month for which the amount remains unpaid up to a maximum of twenty-five percent plus interest as provided in section 32.065.
§ 142.908, RSMo