User enters the UPS shipping charges as vendor expenses, but wants to bill the client a fixed amount (say $10) above and beyond the shipping charge on the expense.
Typically when you record expenses in Core, you enter the markup as a percentage so that the amount charged to the client is the cost of the expense plus a markup. This leads to different markup amounts when the expense cost is different. If you want to charge a fixed markup amount irrespective of the expense cost, you should enter the markup as a dollar amount (say $10) instead of a percentage in the Markup field when recording the expense in the Expense Entries screen. Core automatically reverse calculates the markup percentage giving you a fixed markup amount. Check the Core Help for details.
Note: You cannot enter the markup as a dollar amount if you are entering the expense in the Create Vendor Bill screen. You can only do so from the Expense Entries screen.