You can add payments to the invoice in Moon Invoice easily by following the steps below.

1. Open the Moon Invoice application on your device or tap on this link.
2. From the main menu on your left, go to Sales ➜ Invoices.
3. Open the existing invoice and tap on the Payment icon. 
4. Now choose Add Payment and fill in the details such as payment type, date, and amount. 
5. Click on Save to apply the changes.