In Settings, go to Services and Fees:
Select which service you want to be able to give a discount on, the scroll down to Add Ons:
Create a new Add On with A NEGATIVE value. Unfortunately we can't do percentages right now, only dollar values.
When you go to create an inspection and want to apply the discount,
Select which Service you're providing, then Select the Discount Add on.
Your discount will be reflected in the invoice preview to the left.