Thank you for letting us know what you want in QuickBooks Online.
Currently, the setup for markup and billable expense is through percentage. I'd like your suggestion about entering a flat amount to be introduced to our product engineers. All the enhancement of our products and services are based on customer's feedback.
I've already sent a feedback statement to them. It'd be best if you can also submit a suggestion directly by clicking the Gear icon and choosing Feedback.
Let me know if you have other questions about QuickBooks. I'd be happy to answer them all for you.