Hello there, @nasru.
At this time, the option to set up a discount for each item isn't available. The discount can be applied for each sales transaction. To give you more details on how discounts work in QuickBooks, please check out this link: Add a discount to an invoice or sales receipt in QuickBooks Online.
I can see how convenient would it be to have this feature available for you and your business. I'll surely take note and submit this preference to our developers. This way, they'll be able to access this feedback and put it under consideration.
You can also visit our QuickBooks Online Blog site to be updated with our latest news and product enhancements.
To add up, here some links about managing the products and services including paying sales transactions in QuickBooks Online:
Let me know if there;s anything else that you need by leaving a reply below. I'll be right here to assist you. Take care, nasru.