After month of using nBill, I've tried to improve the ergonomy of my forms.
For instance, I've used mambots tabs, CSS and DIV coding etc.
This took a long time tweaking the orders.html.php file.
Example of tweaked form

And I thought about using templates, like other extensions I've integrated for other projects.
Sobi is one above the others for its great flexibility.

A template file could be attached to a nBill order form.
Where you could put PHP and HTML, with a method to call and display the different form controllers and their parameters by their id.

Hope this idea could make its way !