To mark an invoice as paid, follow these steps:

1. Go to your Calendar
2. Click on the appointment that you would like to mark as paid
   
   

   
   
   

3. Once the appointment is open, click on Invoice (in the left menu)
   
   
   
   
4. Click the Add Payment button
   
   

   
   
   

5. Enter your payment amount, method, and date
6. Click the green Pay button
   
   

   
   
   

Once an invoice is paid, it will be labeled as paid within the Calendar:

Note: If you do not see the "Paid" label, you might need to adjust your calendar zoom settings.