You can send emails from a workflow, directly to invoice recipients on the customer record. See an example of invoice recipients (below).
These can be either invoice recipient or primary invoice recipients. They will appear like this when selecting who you are sending the emails "To" in the workflow (the invoices specific recipients start with the word "Invoice").
As you can see. there are invoice and contact recipients from the dropdown. Ensure you choose the correct recipients for receiving the emails.
This must be an invoice level workflow:
Or
