First you have to make sure that it's legal to do this, as it is not legal in all states. You should contact your state's taxing authority or a good tax attorney to determine whether you can legally do it.
If it is legal, what you need to do is to make an item called something like Processing Fee with a variable price. You then ring up that item for a charged sale. The problem becomes that you need to increase your price by more than the processing fee, since you are now increasing the amount that's being charged, and you will pay more fees due to that.
Judy T Shumway
