When raising an invoice it is possible to automatically have it addressed to the client's Additional Contact (for example a Service Provider).
To achieve this follow these steps:
Step 1: Go to client Details page and navigate to Additional Contact.
Step 2: Click on the Edit button located in front of the contact person. That will open below page. Now click in Primary Contact and hit Update.
Now, all generated invoices will by default addressed to primary contact person.
Please note, If invoice is not meant to go to additional contact person then you can manually choose another contact from the dropdown menu (located under To section)