It would be nice to send an automatic email to the customer when an item that they ordered was placed on back order. Or when an order is split, so.This article explains what demand letters are, when they are useful, and how to write them. Use this letter template to let your customers know the items they've ordered are on back order and will be shipped as soon as possible. When sending an email inquiring about an order that has not yet shipped, it's important to maintain a polite and professional tone. A backorder is generated when an order can't be fulfilled at the time of purchase because the item is not in the seller's current inventory.