Hello, JamesCrozier. 

I'd be glad to let you know that the markup feature is already available in QBO. 

Here's how:

1. Click on the Gear icon.
2. Under Your Company, click Account and Settings.
3. Select Expenses.
4. Click the pencil icon in the Bills and expenses section.
5. Check the Markup with a default rate.
6. Click Save.
7. Click Done.

You can also check the screenshots below:
 

Let me know if you have other concerns. Have a great day!