I have been having similar problems to others re customising the invoice templates.
I followed your FAQ & eventually worked out how to create the woocommerce folder & print-order folder in my theme, copied the templates / print / style.css sheet & then edited it - but could not see any change take effect.
Looked at it & printed through Chrome browser & suddenly working!
How can I get it to work in firefox as well?
(ps - for those with apostrophe problems - solution is to use the html code
' - put it where you want the apostrophe.)