The board of supervisors may make the principal and interest of the bonds payable at the office of the treasurer of the principal county, at such other place within the United States as the board may designate, or at the county treasurer’s office or such other designated place at the option of the bondholders. The place of payment shall be specified in the bonds. The expense of paying the principal and interest other than at the office of the county is a charge against the district funds, to be paid out of the tax for the payment of the bonds.