@rustys_finds
Go to Account Settings>Postage Preferences (under Selling Heading)
You should see a page like this - mine is set up and I can’t recall how, but go from there…

Once it’s set up go into your business policies and in each postage police ensure the Flat postage option is selected if you want it included - about 3/4 of the way down:

someone will be along to explain it much better than me shortly!
Jo