Effective July 1, 1999, payments unpaid on the date on which due and payable shall immediately begin bearing interest as specified in this section. The interest rate per annum for each fiscal year shall be calculated as the greater of the commission's current discount rate or the prime rate plus four percent, each rounded to the nearest whole percent. The discount rate shall be determined by the board of managers on an annual basis. The prime rate shall be the rate published in the Wall Street Journal on the last business day of the commission's prior fiscal year reflecting the base rate on corporate loans posted by at least seventy-five percent of the nation's thirty largest banks. This same rate of interest shall be applicable to all reinstatement agreements entered into by the commission pursuant to section five of this article on and after the effective date of this section: Provided, That if an employer enters into a subsequent reinstatement agreement within seven years of the date of the first agreement, the interest rate shall be eighteen percent per annum. Interest shall be compounded quarterly until payment plus accrued interest is received by the commission: Provided, however, That on and after the date of execution of a reinstatement agreement, for determining future interest on any past-due premium, premium deposit, and past compounded interest thereon, any reinstatement agreement entered into by the commission shall provide for a simple rate of interest, determined in accordance with the provisions of this section which is not subject to change during the life of the reinstatement agreement for the future interest. Interest collected pursuant to this section shall be paid into the workers' compensation fund: Provided further, That in no event shall the rate of interest charged a political subdivision of the state or a volunteer fire department pursuant to this section exceed ten percent per annum.