Welcome to the Community. I can help you add a discount option to your invoices in QuickBooks.
There are two options you can try to apply discounts to your invoices. You can either include discounts per line item or directly to the total amount.
To enable item-wise discount or total discount:
For more information, as well as how to show the Discount column in the printed invoice, I recommend the following article: Discount by line item on invoices and sales receipts.
Drop me a comment below if you have any other questions. I'll be happy to help you some more.