Product field

Each line item starts out by selecting the product. You can add products on the fly by choosing Create and Edit... from the bottom of the list. Once there are more products in the list, you can also bring up a product search window using the Search more... option. After you select the Product field, Odoo retrieves the tax and pricing information from the server to display in the line item.

In Odoo 11, the total is automatically updated as you add new line items. Still, Odoo provides an update link next to Total on the form. You should consider hitting this if you have any doubt as to the accuracy of the total.