It's good to see you here in the Community, office161.
I'd be happy to help you send paid invoice to your customer. You can go to the Invoices page and send the paid invoice from there. Let me guide you how.
From the left menu, click Sales and choose the Invoices tab.
Find the invoice that marked as paid.
Under the Actions column, click the drop-down and select Send.
Enter the email address and hit Send.
Or you can go directly to your customer's profile and send the invoice from there.
After following the steps above, you can send the invoice marked as paid with zero balance. You can get more details about sending invoices in these articles: