Each county issuing revenue bonds hereunder may invest any funds in bonds, notes, certificates of indebtedness, treasury bills or other securities constituting direct obligations of the United States of America; in certificates of deposit or time deposits constituting direct obligations of any bank as defined by the Idaho Bank Act, provided, however, that investments may be made only in those certificates of deposit or time deposits in banks which are insured by the Federal Deposit Insurance Corporation, if then in existence; or in short term discount obligations of the Federal National Mortgage Association. Any such securities may be purchased at the offering or market price thereof at the time of such purchase.