What does Paidlabs.com do that stripe doesn't? Stripe does invoicing AFAIK.
(Paid is a YC company, btw!)
EDIT: My apologies for the misunderstanding! I didn't realize that you've already built that functionality from your top comment.
One of our technical needs was auto-charging clients for overdue invoices that were neither approved nor disputed. That's application logic, not something we can use an external system for.