You can do this is by charting a service item (i.e DISCOUNT)
Either give this service a permanent discount fee value or double click the charted service to amend the fee. Add a minus in front of the value (i.e -150.00) then click Ok.

When you print a treatment plan/estimate it will now show the discount separately

PLEASE NOTE THAT ALL OTHER TREATMENT WILL NEED TO BE CHARTED AS YOUR STANDARD PRICES