Hello there, Kirby.

You can go to the Invoice page and send the paid invoice from there. Let me guide you how.

1. From the left menu, click Sales.
2. Choose the Invoices tab.
3. Find the invoice that marked as paid.
4. Under the Actions column, click the drop-down and select Send.
5. Enter the email address under TO section.
6. Click Send.

After following the steps above, you can send the invoice marked as paid with zero balance.

You can read this article for your reference: How to Email or Print an Invoice.

You can always leave a comment below if you have any other questions in sending an invoice. I'll be here should you need further assistance.