You're on the right track, @Serge01!
As of the moment, there isn't an integrated way of tracking all discounts for your bills from your vendors. What you have been doing is the only option available, creating an additional line item as the discount for your bills.
But don't worry, I'll pass along your request to our product engineers so they know of your requests. Feedback from our customers, like you, help us improve our software's functionality to offer better services.
In addition, here's an article you can read to learn more about our latest functionalities and improvements: QuickBooks Resource Center.
Lastly, I've got you these few articles for ideas in managing your vendors and expenses: Manage Suppliers and Expenses for QuickBooks Online.
If there's anything else that I can help you with, please let me know in the comments down below. I'll be happy to help.
