A client wants to add an x% service charge on an invoice where the customer is paying by credit card, as a separate line on the invoice (and flowing to its own GL account).

What's the best way to go about implementing this? I thought about commandeering the sales tax function, but they already charge sales tax.

Something tells me this is a basic question that many people other than me know the answer to, so I hope I won't feel too stupid.

Steve