We do not charge additional freight for back orders shipping within the continental United States.  Freight charges are included in the original order total. If you chose to ship your order via collect account, the back-ordered units will ship on the same account.  We are not able to ship back-orders outside of the continental United States, so if you have items to order that are out of stock and you are located out of the continental United States, please either choose to ship complete or place a separate order for these out of stock items.