X-Cart is integrated with the Royal Mail (the national postal service and courier company in the United Kingdom) via the add-on Royal Mail Service.
The Royal Mail integration add-on for X-Cart sends the order information to Royal Mail. There it is given a tracking ID, shipping, and customs labels, after which RM sends it back to your store. After that, you can label the orders and print out the manifest for the courier.
To start using Royal Mail Service with your X-Cart store, make sure the Royal Mail Service add-on is installed and enabled as described in Installing Addons from X-Cart App Store.