I understand. All all-in-one app are targetting teams. I want to change it. I want to work for freelancers. I will maybe have less users and that's ok. Thanks for your comment.
Sadly, my portal already exists, and is deeply connected to Stripe and my way of doing business. So, I don't ever plan on doing what this person is doing. But I do highly suggest integrating it with Stripe Connect and make the project entirely free, and only charge a % of invoices total (like 1-5%). If it is free, people will use it. If they can send an invoice and automatically charge a client (assuming the client saved their billing information), that is money in the bank and you don't have to worry about collecting or them paying late ... that is worth it's weight in gold, as a contractor.
Basically, I don't see a value-add (YET) for paying for something when right now this admin work isn't costing me any significant time or causing me issues.
Maybe a model like this would work for you as well? The users you are replying to might switch and sometimes pay if they have a busy month.