Hey there evialls,
Thanks for reaching out here. It's important that you are able to charge the correct amount to your contractors. QuickBooks Online is a tool to to have, it helps keep your books balanced so you don't have to do all the work. I can provide some information about percentages on products and services.
There isn't an option that allows you to set a limit on amounts for products or services in an transaction, or calculate sales based on a percentage. I can see how this feature would be beneficial for you to have. I recommend leaving feed back by navigating to the Gear icon and selecting Feedback. These comments are considered when our Engineers are developing new features for QuickBooks Online.
If you have any other questions, feel free to reach out here.