For exports, I need to set up a Commercial Invoice (not a Tax Invoice) which shows weight and country of origin for each line item. How can I add two extra columns?

Hello Guy W, 

Welcome to the Community page, 

So there is no way of being able to add extra columns into the invoice section, all you can do is wrote it in the description or product name section so it shows up for you on the invoice. You can leave feed back in the main cog wheel about it, this gets sent straight to our developers.