How can I add a percentage uncharge (rush order) to an item?

[bythelakedesign]

I am wondering if there is a way to charge a "rush charge" to a customers item... it would need to be a percentage of the item cost....

[ACAcatering]

You could add a modifier to the items.  You could then choose the modifier when you need the upcharge.  It may be a bit time cosuming as you would need to figure the amount for each item and set it as modifier.