Self-Cleaning design Springs securely closed Full 100% unrestricted opening Easier to open and close. Shipping Policies for Premium Ductwork: All premium ductwork is subject to a 20% shipping charge with a minimum of $15 shipping charge for all premium ductwork on your order. You will be notified by our customer service department of the amount of your shipping before we charge your credit card. Note that certain ductwork items are drop shipped from the manufacturer and may not be shipped with the rest of your order. Because of its large dimensional volume, ductwork cannot be shipped by air.