Is it possible to be profitable (in a successful startup sense, not casual/bootstrapped/side project sense) at that low level? Maybe I'm missing something but I don't see a lot of room for revenue growth here. I'd figure that the average freelancer as a handful of contracts a year so you're looking at $10-20/year per customer.
I'm not saying the idea itself isn't good. Anybody that's freelanced or done corporate invoicing knows the annoyingness of "We'll pay you within 30 days of deciding to pay you"[1]. Solving that problem would benefit a lot of freelancers. I just don't see this being a sustainable solution in the form that's being presented.
[1]: Quote is shamelessly stolen from a patio11 comment: https://news.ycombinator.com/item?id=9076590
Edited to add link to quoted comment