It's a cool project but it is 'niche'.
I think the purpose of the site is more about the alternatives to 'large players', platforms and infrastructure companies. Still Constantin Graf should have clarified out of politeness but possibly he's busy or doesn't have time to respond to every email.
However I'd point out there is a market for European 'Product Hunt' that would include more of these smaller projects.
I don't think creating an invoice is "niche". It is such a common need for users that invoicing software should be included in the operating system application suite. (Which it is somewhat if you consider Pages invoice templates).
Millions and millions of people need to make and send invoices. Many more than people who need domain name registrars, uptime monitoring services, content delivery networks, or microblogging services.
Thanks for the comment. I hadn’t thought about this before, but it makes sense - I agree.
About European Product Hunt - very good idea.
I was thinking recently that we need more European social networks, messengers, etc.
It’s a very good time to build imo =)