By german law were are supposed to print the unique customer id ( number ) on an invoice as an identifier for tax office. However on the very most invoicing application this is not provided. On invoice plane as for now it’s very easy implement as a current fix by anyone who needs it.
Just access the template on your ftp server located at:
application/views/invoice_templates/pdf Kundennummer <?php echo $invoice->client_id; ?>
i.e.
And add that line to where you need it.
However I’d still like to see it being built in. For example there could be an option to activate it or similar.
Best regards.