The send email action ... it appears the Sender needs to be an entity? Tried adding an email address field to the invoice, but it's only available as a Recipient, not a Sender. Is there any way around this without creating an employee record? The client just wants it to be from a static billing address.

As far as I know - must be an employee

We have had to make dummy employee records for this. So we have like Accounts Payable and Accounts Receivable as employees to be able to use in workflow emails

We ended up doing the same.