After this Shopify app's installation pre-order button with beautiful title shows up on product's page that is out of stock. Customer clicks on this button and goes through a regular order procedure, which allows to sell even if a product is out of stock. Title near the button will show a message that can be edited by you, for example: "We will fulfill the item within 3 days". Full list of pre-orders is shown in admin panel of the app, so you can easily control and handle such orders.