Invoice processing is a continuum, from all-paper and manual to fully digitised and automated.
On one end, too much time, money, and frustration are involved in fixing errors, handling exceptions, and fielding supplier calls.
There are of course many applications to produce invoices. Many eCommerce clients will already have their invoicing built-in to their eCommerce application.
However, for the others, who may be thinking of a manual system, here are just a few links to relatively inexpensive online invoicing packages.
There are hundreds, if not thousands of these available, and this list is just a taste.
I do not endorse any of these, do not recommend any in particular, and and have not used any of them........ this is just for your information only.
So, here are the few......