The issue I am finding with even calculating the fee and adding it as a line item on the invoice is that you need to know before making the invoice how that customer is going to pay. I find some customers usually pay with check, but because the option is on the invoice, they sometimes pay via credit card. So I cannot determine ahead of time exactly how the customer is going to pay. What would be so great would be if we send the invoice as normal, but when a customer pays via credit card, have the payment page add the fee at that time, then take your fee amount and pay us the invoice amount. Simple and headache free!
