How do I create a coupon item to add a discount to an invoice?

1. Go to Menu > Settings > Products & Services > Services.
2. Create a new procedure code and give it a name.
3. Enable the item setting to exclude from percentage discounts.
4. Set the price as a negative (-) value. 

If you want to be able to change the coupon amount, you can also enable "allow price change" on the item.