1. If the tax imposed by sections 287.690, 287.710, and 287.715 are not paid when due, the taxpayer shall be required to pay, as part of such tax, interest thereon at the rate of one and one-half percent per month for each month or fraction thereof delinquent. In the event the state prevails in any dispute concerning an assessment of tax which has not been paid by the taxpayer, interest shall be paid upon the amount found due to the state at the rate of one and one-half percent per month for each month or fraction thereof delinquent.

2. In any legal contest concerning the amount of tax under sections 287.690, 287.710 and 287.715 for a calendar year, the quarterly installments for the following year shall continue to be made based upon the amount assessed by the director of revenue for the year in question. If after the end of any taxable year, the amount of the actual tax due is less than the total amount of the installments actually paid, the amount by which the amount paid exceeds the amount due shall at the election of the taxpayer be refunded or credited against the tax for the following year and in the event of a credit, deducted from the quarterly installment otherwise due on June first.