Thank you for your recent Order No. XXXX dated MM/DD/YY for (Product and quantity).
We shipped the (Product that as shipped) today; however, we are currently out of (Product that is backordered). We have placed it on back order.
The (Product that is backordered) can be shipped within the next XX weeks or, if you prefer, a refund for $0.00 can be processed. I have enclosed a SASE so you can inform us of your decision.
Thank you for your business. We apologize for any inconvenience this may have caused you.