[ADD] inventory: How to invoice the shipping cost to the customer ?

https://docs.google.com/document/d/1cdu_uES8QovYtudttIC9OtMZqushsOx6VaicsF4fXP4/edit?usp=sharing

inventory/shipping/operation/invoicing.rst

@@ -1,3 +1,88 @@
 ==================================================
 How to invoice the shipping cost to the customer ?
 ==================================================
+
+Overview
+========
+
+There are two ways to invoice the shipping costs:
+
+1.  Agree with the customer over a cost and seal it down in the sale
+    order
+
+2.  Invoice the real cost of the shipping.
+
+Configuration
+=============
+
+To configure the price of your delivery methods, go to the **Inventory**
+app, click on :menuselection:`Configuration --> Delivery --> Delivery Methods`.
+
+You can manually set a price for the shipping: It can be fixed or based
+on rules.
+
+Or you can use the transportation company computation system. Read the
+document :doc:`../setup/third_party_shipper`
+
+How to invoice the shipping costs to the customer?
+==================================================
+
+Invoice the price set on the sale order
+---------------------------------------
+
+On your sale order, choose the carrier that will be used. Click on
+**Delivery Method** to choose the right one.
+
+.. image:: media/invoicing06.png
+   :align: center
+
+The price is computed when you **save** the sale order or when you click on
+**Set price**.
+
+To invoice the price of the delivery charge on the sale order, click on
+**Set price**, it will add a line with the name of the delivery method as
+a product. It may vary from the real price.
+
+When you create the invoice, it will take the price set on the sale
+order.
+
+.. image:: media/invoicing01.png
+   :align: center
+
+Invoice the real shipping costs
+-------------------------------
+
+On your sale order, choose the carrier that will be used. Click on
+**Delivery Method** to choose the right one.
+
+.. image:: media/invoicing05.png
+   :align: center
+
+The price is computed when you **save** the sale order. Confirm the sale
+order and proceed to deliver the product.
+
+The real shipping cost are computed when the delivery order is
+validated.
+
+.. image:: media/invoicing02.png
+   :align: center
+
+Go back to the sale order, the real cost is now added to the sale
+order.
+
+.. image:: media/invoicing03.png
+   :align: center
+
+When you create the invoice, it will take the price computed by the
+carrier.
+
+.. image:: media/invoicing04.png
+   :align: center
+
+.. note::
+    If you split the delivery and make several ones, each delivery
+    order will add a line to the sale order.
+
+.. seealso::
+    * :doc:`../setup/third_party_shipper`
+    * :doc:`labels`