@baaaas: well, for our need I only had to remove this big block with total amount above the details/item table. Definitely a nice eyecatcher, but for my partner it was too dominant; so, I just removed it (in the copied body template) for our need. if we will have future enhancements or changes on the invoices, I will share them with you. if this is worth an option / setting in the admins area – I doubt – as it can be easily managed by the templates …
one improvement (you might already have built in) is worth to mention: the textareas in the admin settings area could support the built in wp_editor. Advantage: preformatted text (example: bold company name, italic phone numbers etc, links (hrefs)) … but may be this works in the premium or newer plugin already ;-).