Hi kndzpntnr,

I can see how convenient to be able to add a mark-up on the Estimate.

As of now, this option is still unavailable. That said, I'd suggest sending another Feedback to our Developer's team. You can do this by clicking the Gear icon and then selecting Feedback. Enter your suggestion, and click Next. 

Then, you can track feature requests through the QuickBooks Online Feature Requests website

As a workaround, you can create a service item and add it to your estimates.

Also, whenever you're ready to create an Invoice out of the Estimate, you can check this guide: Convert Estimate to an Invoice.

Feel free to get back to this thread if you have other concerns.