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.
My feedback as a freelancer: one of my main goals is to keep my costs as low as possible, especially fixed costs. For tracking time and generating invoices I use Google Sheets, this has served me well for many years. My invoice is a tab in my timesheets that I manually enter a month's hours into as a line item, and VAT is calculated, I then bump the invoice number and date and export to PDF. This whole process takes a couple of minutes, and I like that some parts are manual because I want to be able to doublecheck everything first.
I think where this kind of tool would really start to be useful is for people expanding from being a one-person operation to more consultancy/agency operations. So I would really target people who aren't working alone by default if I were you.