The Simple Invoice software supports discounts to individual items as well as to the total of the invoice. To add discount for particular item use discount column. You can use only fixed amount, percentage discount is not available.
If you want add discount applied to the total of invoice you can use Custom Fee feature. For exaple you need add 10% discount to Invoice, first customize in fees:
Then use button to add discount to invoice:
Discount will appear on invoice in summary: