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 that 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”.