Printinvoice.pl should be able to load custom CSS like printsplip.pl does. I think both these prints could share the same systempreference ClipCSS. Please comment.
The mentioned system preference is named SlipCSS, of course. Sorry for my typo.
After a discussion with black23@gmail.com changing this topic to allow full customization of printinvoice.pl. This function should read the system preference SlipCSS as stated before. In addition, content customization through notices and slips at /cgi-bin/koha/tools/letter.pl should be allowed the same way as ISSUESLIP and ISSUEQSLIP already allow. I propose a new letter code: PRINTINVOICE.
*** This bug has been marked as a duplicate of bug 22761 ***