Any employer who has been assigned an experience rate, and who has filed all reports required under chapters 42 — 44 of this title, and has paid all contributions, interest, and penalties due under chapters 42 — 44 of this title, may make a voluntary contribution to his or her account. Such voluntary contribution shall be paid not later than thirty (30) days after the date on which the department has issued a notice of the employer’s experience rate, or prior to the expiration of one hundred twenty (120) days after the start of the calendar year, for which the experience rate is effective, whichever is earlier. Upon timely payment of a voluntary contribution, the contribution shall be credited to the employer’s account balance and that employer shall receive a recomputation of its experience rate for that calendar year. No voluntary contribution shall be refunded in whole or in part.