Click on a reservation
Click on the number of the Reservation
Click here
Make sure your invoice is Opened
And click on Save
Go back to your reservation
Click here to Add a Payment
Select the payment method
Click here to pay the full amount
Click on Add
Click on Add to invoice
Click here to select your reservation
Click to select the client's reservation
Click here to add your payment
And click on OK.This is how you can add a Payment to a created invoice.