The invoices produced by nBill are template-driven.  It is therefore possible to format your invoice to get whatever look-and-feel you want, and you can use a different template for different vendors if you are making use of the multi-company feature.

Invoicing templates should only be created by PHP developers. If you do not know PHP, you will have to either use the default template, or get a PHP developer to create a new template for you. Full instructions for creating an invoice template are in the comments of the default template file (/components/com_nbill/templates/default/index.php), and on the nBill website: https://www.nbill.co.uk/support/invoice-templates.