Follow these steps:
Open the CRM module from the main page and navigate to the CS tab
Locate the relevant guest and click on its ticket to open the guest chat
Open the reservation page (Expand the chat's side menu and click on the square icon with an upward arrow).
Click on the "Add Invoice Item" button under the Finance section
In the "Type" field select whether to provide a discount or add an additional charge
Enter the amount
Add a description
Click save.