If a reservation is already made and the guest wants to add a few extra's in the form of add-ons, you can easily add these to the system. 

Click on the reservation in your calendar

Click on the add-on field under the pricing on your right hand side

Add the add-on here, press save

The add-on can be a manual add-on, or a pre-saved add-on. If you intend to use certain add-ons more frequently we would advise you to save the add-on in your system