When a product is receipted in there is an option to Add Shipping Costs to Products. When a product is added to an invoice, it will use the Latest or Average Landed Cost.
When the product is added to the Invoice, it will use the Latest Landed Cost against that product at the time.
If the Shipping cost is added after the product is added to an Invoice it will not be included, unless the user right clicks on the product code and opens, check the costs and if the shipping is included, then reset the product on the invoice.
On the exampled below, the first line was prior to the Shipping Cost being added. The second line is after is added and the product is entered again or right clicked and updated. Note the difference in the markup.