Hi there, Happygirl!
In QuickBooks Online, we have a feature called Projects. You will enter your invoices and your expenses per project, and in Project section, you can automatically see your profit margin for each of them.
To enable Projects:
Once enabled, the Projects menu will be added to the left navigation bar of your Dashboard. Then, just click on it and click New Project to add one. If you like to add transactions to the project you added, simply click on the Add to Projects button and choose the drop list.
Here are some articles for your additional reference:
Feel free to post again when you need help.
Have a good day!