I think you've hit a really nice market niche too. If you get a support call it's from someone having difficulty extracting money from someone else because invoicing isn't working. So long as you solve that problem for them they'll associate your software as being directly valuable to their bottom line, and that'll make them much more likely to agree to pay money for it.
I suspect that approach wouldn't work nearly so well if the connection between the software you're selling and the money rolling in to their bank account wasn't quite so obvious. I probably wouldn't try such a generous free version of, say, wiki software or document management or backups - things that are way too easy to consider as "cost centres"...
On the flip side, you can get very angry customers calling you and saying your broken software is preventing them from making money, or even worse, losing money. When I worked for a niche shopify like business this made for not fun emergencies. Even less fun if we made their POS not work.
I suspect that approach wouldn't work nearly so well if the connection between the software you're selling and the money rolling in to their bank account wasn't quite so obvious. I probably wouldn't try such a generous free version of, say, wiki software or document management or backups - things that are way too easy to consider as "cost centres"...