To contract for the performance of any work in connection with the establishment, maintenance and operation of any airport as it may deem to be in the public interest, provided that contracts requiring the expenditure of $2000.00 or more by the County Airport Commission shall be let to the lowest responsible bidder at a public letting held after due publication of notice thereof, however, this requirement shall not apply to contracts for professional services if the approval of the award has been given by the Department.