this site may be able to give you a better price and not need to pass on the fees to your customers they definitely integrate directly with TSS

an inventory item has on the pricing tab

a check box for dynamic pricing which when you add it as the last item on the invoice it will help you recover the processing fee or percentage the credit card company is assessing.

Do not make the item nontaxable because it will not recoup the percentage of the taxes if you do.  

So in the inventory create an item call it cc fee and maybe the model can include the percentage number

and down in the bottom of the pricing tab you can set the percentage that your credit card processor charges your company.

Sales people can ask customers how they are paying,  and add this item as the last item to your invoice.