(a) A participating municipality that fails to remit before the 16th day of the month all contributions required by this subchapter to be made and remitted to the retirement system by that date shall pay to the retirement system, in addition to the contributions, interest on the past-due amounts at an annual rate that is the total of the system's investment return assumption for the preceding calendar year, plus two percent. The retirement system shall notify participating municipalities of the rate of interest that will be due on late payments.
(b) Payment is considered timely made if transmitted by first-class United States mail, postage prepaid, and postmarked not later than the 15th day of the month in which the payment is due.