Hi propropro,
It's indeed helpful to have the option to enter discounts on bills and other vendor transactions, like in QuickBooks Desktop. While we're unable to do this in QBO, I recommend sending feedback to our product development team. That way, they can consider adding the feature in future updates.
In the meantime, you can create a discount item and add it to invoices like what my colleague, katherinejoyceO, shared above. She also shared about entering a vendor credit, which can be an alternative to a discount item.
Feel free to go back to this thread if you have more questions about entering vendor discounts in QuickBooks. I'm just around to help.