The problem with the solution proposed by DarellA is that you  would need to know if your client intends to pay by credit card in advance of creating the invoice. That's impracticable in most cases, If you want to send an invoice that gives the client the price with a notice that they can pay online via credit card, but will have to incur a 3% convenience fee, the above solution doesn't work since the total for the invoice would include the 3% service item fee.