How do I mark a printed invoice as "sent"?

Hi there, @ngood. 

Thank you for posting to the Community. 

Let me walk you through on how to mark an invoice as sent.  

You would need to send the invoice to yourself in order to change the status to Sent. This is the workaround we can use to get the outcome you wanted. 

Here’s how:

1. Go to Sales from the left menu and select Customers.
2. Choose the customer by clicking it.  
3. Locate and open the invoice.
4. Enter your email address on the right side of the customer name.
5. Click Save and send.
6. Hit Save and close. 

Once done, you’ll find the invoice marked as Sent. 

You may check out this article for your reference: Send an invoice.  

If there's anything else I can do for you, please let me know. I'll be around to help you. Have a great day!