The commission shall have authority to advertise for bids for the purchase of any materials or things and for letting contracts for furnishing any labor and material for carrying out plans that may be approved for the beautification and landscaping of any part of said parkway. No purchase or contract which is to be paid out of county funds, shall be final or any such contract binding upon the county, until accepted by the board of supervisors by order spread upon their minutes. Any such purchase or contracts that are to be paid for out of funds provided by any other governmental department shall be made and accepted in such manner that may be approved by the governmental department providing such funds.