Except where the public moneys of a depositing unit in the custody of the treasurer at any one (1) time are less than one thousand dollars ($1000), the treasurer shall deposit, and at all times keep on deposit, subject to the provisions of this law, in designated depositories, all public moneys coming into his hands, and it is hereby made the duty of said supervising board not less than once every six (6) months to certify to the treasurer the capital and surplus or reserves and unallocated or undivided earnings, as applicable, of each public depository, a copy of which certificate shall immediately be served on the treasurer by the supervising board or its clerk; provided, that with the approval of the supervising board of the depositing unit, the treasurer is authorized and empowered to invest surplus or idle funds of the depositing unit in investments permitted by section 67-1210, Idaho Code, and interest received on all such investments, unless otherwise required by law, shall be paid into the general fund of the depositing unit: and provided further, that as to all public moneys in the custody of the treasurer of a depositing unit for which there is no legal depository available under this chapter, it shall be the duty of the supervising board of the depositing unit to designate and place for the safekeeping of such public moneys, and until such designation it shall be the duty of the treasurer to deposit such excess sums on special deposit in any public depository, and the expense of such service shall be borne by the depositing unit.