Something that may be of use to others is the ability using the translate facility to modify the ini file to relate to different titles and phrases.
Some items in my setup do not require certain field tiles such as fiscal number of Invoicer _url, I have changed these for my purpose to create not invoices but advice notes as we pass on our data to head office to invoice. Even the title Invoices on the menu is now called Advices. This software still retains the underlying references, but using the modified language file really is so useful.

One feature that may be useful to others is a second contact option, some business have a need to show two contact details on a form, such as in shipping invoices which show shipper and consignee, currently I have static code for the Advice header, and use the Invoicer details for the shipper and the Receiver address as the Consignee. This works adequately, but maybe something for the future.
I have also used the Fiscal Number for a Job Reference number, which heads the page.
One feature that would be valuable to me and possibly others is a user field that could also be used in the email header. I cannot find a way of changing any variable from {invoice_num} in the email subject to another variable for some reason, I tried the fiscal number which would translate to job number to no avail.

One thing I have found is that all my past problems were not related to the software itself, mainly my issues were to do with not having a compatible setup.

There are some strange issues with the visual setting of templates, but we are warned not to use that option if possible. I consider myself quite better at understanding this.

NOTE
I made the mistake of putting the com_invoices language file in the Administrator/Language directory, and it didn't work, so I put it in Language directory and it worked.

I Hope this helps others and I am happy to pass on my new knowledge.