The additional contributions of such members shall be paid by regular monthly salary deductions in the amount specified by the member, but in no case less than six dollars ($6) per month. The total amount due shall be paid prior to the effective date of his death or retirement, except that his spouse, or his minor children if no spouse survives him, may elect to pay any balance due within 90 days after the death of a member. If the total amount due is not paid the member, his spouse or his minor children shall receive credit for service as a safety member for only the time for which such additional contributions are made. Any member who has elected to make such additional

payments in installments by salary deductions may complete payment thereof by a single lump-sum payment at any time prior to the effective date of his retirement.