Go to Discounts to add new discounts, or update existing ones. You can configure different discount types (fixed or %), set the value, and write descriptions about them so the entire staff knows how to use them correctly.
Clients can have discounts associated so every time you create an invoice for the specific pet parent the set discount will be automatically applied. You can remove the discount or change it to a different one if necessary.
To set a client discount or update an existing one, simply go to the client profile and look for the discount field.