Can this be used, or setup in a way that supports the standard invoicing model?<p>i.e. I want an api call to email a .pdf to a customer, and then have them &quot;click this link and enter your info to pay via checking account&quot;. Several kinds of customers, including consulting, would be much happier with a &quot;push&quot; model of paying, than a &quot;pull&quot; model.