Dear …
RE: __(Type of Item)__ - On Back Order
We are in receipt of your recent order for a __(make/model of item)__. Thank you, we appreciate your business.
Unfortunately, we regret to advise you that this item is currently on back order and we are unable to ship it to you at this time. Delivery date will now be extended by approximately __(length of time)__. If, as a result of this delay, you wish to cancel your order, please contact us at __(contact information)__ as quickly as possible.
We apologize for any inconvenience this delay may have caused you.
Sincerely,