we have used freshbooks — as it is a leader and has an API, but it&#x27;s obviously not built to be used for lo&#x27;s of automated SaaS invoices. some more here: <a href="http:&#x2F;&#x2F;blog.fortrabbit.com&#x2F;battling-billing" rel="nofollow">http:&#x2F;&#x2F;blog.fortrabbit.com&#x2F;battling-billing</a>