Use additional surcharge charts to create a discount, and have it displayed on the invoice as a line item on the order. This is an alternative to reducing the base rate.
Step-by-step guide
Go to Maintenance > Rate Charts.
Click on Surchages.
Add a new surcharge, click the down arrow next to "New" then "Rate Chart Menu" > "Surcharge Rate Chart", or find and double-click on the surcharge rate chart you would like to apply the discount to. See Surcharge Rate Charts - New / Edit Surcharge Rate Chart.
Create an item called "Discount" on the items table. See Items.
Click Add New button to add a new item. See Surcharge Rate Charts - New / Edit Surcharge Item.
Select "Discount" in the "Item" field.
Enter a negative amount for the "Rate". This can be either a percentage or a fixed amount. If your discount is a percentage, set your "Min" to a large negative number.
