Yes, you can add comments in a closed invoice by accessing it either from the Appointment Book or from the Admin module. However, you must have the required permissions to edit a closed invoice.
To access a closed invoice from the Appointment Book:
Click an open slot, and then click Search Invoice No/Receipt No.
Click Show.
Select I want to>Edit Invoice.
Click Edit.
Add your comments in the Comments section, and then click Save.
You can also click the appointment whose invoice you want to edit, and then click Show Invoice.
To access a closed invoice from the Admin module:
On the Admin dashboard, click Report>Sales>Select a Report>Search by Invoice No.
Enter the invoice number and click Search.
Click Edit.
Add your comments in the Comments section, and then click Save.