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Welcome to the Community space, @david152.
Adding a commission rate in QuickBooks Online can be done if you create a service item. Once done, you can add this item to your invoice or bill depending on how you get the commission.
Here's how to create a service item for the commission:
Here's how it'll look like when you create an invoice.
If you want to track sales commissions, you can check this article link: Track sales commissions in QuickBooks Online.
Let me know if you have other questions. I'm always around to help. Take care and have a nice weekend.