You can add a discount to the subtotal of the estimate or invoice and it will be labeled clearly for the client to see as Discount on the saved document.
Note: The naming of this discount label cannot be changed.
While you can’t apply a discount to individual line items, you can price the line item at the discounted rate, then include a line in the description of the item about the discount.
For example, for a 10% discount on a $100 item, price the item at $90 and add in the description “10% discount applied, regular price $100”.