Just figured this out. For example: Make a line item as cc fee or whatever you want to name it. Description 4%. Then charge $0.04 and leave the quantity blank. Then save it. Create an invoice as you normally would. Once you have all of your normal line items put in, click the add subtotal so it will show the customer what the amount is before the card fee. Then add another line item, which would be your card fee. Now for the quantity, enter the subtotal amount. It will add the fee on the end. Simply put in your template that if you wish to pay by check, to just pay the subtotal amount. Hope this helped.