To add a discount, click +Add Items on the top right of your builder window.
In the overlay window, click +Create New Items.
In the next overlay window, add your company name as the manufacturer and type in Labor as the product name (you can type in whatever works best for you). In the Price field, put the negative amount that you wish to add as the discount. Then click Add and Close. The amount will be subtracted from your total.
Note: Adding a discount for labor works the same way.