Adding the shipping & freight cost to the invoice is possible by adding the 'Shipping Cost' as a new item to the invoice. Here's how you can go about doing this:

• First you need to add a new item as 'Shipping Cost' with a default rate. (Steps to add an item)
• Then you can select this item as the last line-item for the invoice. You can even edit the rate manually based on the cost of shipping.