One of the features for PrestaShop is its ability to take an order that has multiple products, where one product is in stock and another is out-of-stock.  Prestashop splits the order into two different orders to allow the in-stock item to immediately ship.  This feature is enabled under an Order option called DELAYED SHIPPING.  The following tutorial will guide you in enabling this option in the Prestashop 1.5 Administration Panel under the Order preferences.

How to Enable Delayed Shipping for Orders in Prestashop 1.5

1. Login to the Administration Panel as an Administrator
2. Hover over the menu bar where it says PREFERENCES, and then click on ORDERS in the drop down menu. Look for and then select the DELAYED SHIPPING option. The screenshot below illustrates its location:
   
   
   
   
3. Select YES to enable to the option or select NO to disable it.  NOTE: When this option is enabled, any order that has an item(s) not in stock will be split into two different orders. Each will have shipping and payment requirents like a normal order. The order with the items not in stock will be delayed until the order item becomes available in stock.
4. Select the green SAVE button in the upper right hand corner to save the option selected.

Allowing delayed shipping allows your customers to be immediately satisfied with items that you have in-stock, while preserving orders of items out-of-stock for a later delivery date.  This enables your customers to use your shop without having to leave for another to purchase the item elsewhere.