Once an invoice has been fully paid, you can send a thank-you message to your client.
Select an invoice
Make sure it's fully paid
Click on ...
Select Send a Thank-you message
You can change the wording or recipient of the message
The text of the thank-you message can be pre-defined as well. Navigate to Settings - E-mail templates & settings - Thank-you messages.
You can use smart tags such as #CLIENT_NAME# or #TOTAL# in your message, which will be replaced by the real figures later when sending the message.