From some months now, I'm working on a minimalist solution for my own invoicing needs. I thought it was better to push this forward, and to finish the job: I've made https://fossbill.org
Fossbill is a simple billing tool. It allows to manage customers, products, and bills. We values efficiency, reliability, and accessibility.
I think it would be great if you could add more screenshots to the documentation. Beyond the landing page, I couldn't see how the bills look once generated, or how to generate one.
You right, I didn't included screenshots of the generated PDFs yet. Here what it look atm for a draft, then a bill.
As you can see, it is still quite minimal. I intend to add (1) editable user styles, and (2) brand logos you could add to your user account, and to your customers.