Just a few UI suggestions - hope you don't mind a dot-point list
Add a line item be a button, or something a bit more obvious?@engram-design I did a PR for your last point as its something I often need too: #1050

Coming from Commerce 2, the Commerce 3 interface seems very clunky. I love the VueJS aspect of it, and how everything is editable in the order etc. That is amazing. But I feel the whole UI for the line items and order history is not clear:
@bymayo Thanks for the great feedback, we definitely have plans to improve the order edit page, including most of what you mention. Stay tuned to this issue!
There have been a number updates to the Edit Order page as of the release of 3.1.
I will close this issue in favour of making new specific issue related to the current version.
Thanks!