Hi there, @truecraftconstru.
Allow me to help share some insights about adding a mark up for invoices in QuickBooks Online.
In QuickBooks Online, markups will only take effect in invoices are regenerated from a billable expense. Since you have a 3rd party invoicing, you can create a service item and manually add it as a mark upon the invoice.
Here are some good references that you can check out about billable expenses for your future use:
However, our developer and continuously working to improve our product to meet our customers' needs. That said, I'd encourage you to visit our QuickBooks Online Blog to be updated with our latest news and updates including product improvements.
Don't hesitate to let me know if I can be of additional assistance. Cheering your business to continued prosperity.