Records that you delivered an issued or sent invoice to the patient yourself, such as in
your own email. Hint sends nothing: no email or text, no payment and no change to the
invoice's amounts or dates. To have Hint email the invoice, use Email Customer Invoice instead.
An issued invoice becomes sent. last_sent_at keeps the latest send: an earlier or equal
sent_at succeeds without changing it. Without sent_at, the request records a send now, so
when you retry a request for the same delivery, send the same sent_at.
Fails with 422 when the invoice is not issued or sent, or when sent_at is not a
valid date-time or is more than a minute ahead of Hint's clock.