You could use a script that checks to see the payment type and adds a line for the credit card fee with the 5% fee. You would need Elliott to help on this. At the Sage Summit a similar script was demoed by Elliott I believe in which if the order was over a certain amount a line was added for a free hat.

I wonder if the order discount % could be set to a negative amount...

Setting up something based on business objects might be difficult but this might be a fairly simple button script (grab the header totals then write a new line with the 5% amount).

If you are talking about an item that is past due and then they want to pay it with a credit card, don't you want to apply the 5% fee at the time of the cash receipt? If so, you just want to add it while doing the cash receipt entry as a miscellaneous line charged to a g/l account as opposed to adjusting the original invoice? That's what I have one of my client's doing now.

If I recall correctly, they are just manually adding the extra line, but it seems like it would be a relatively simple button script to compute the additional amount.

The other thing that you have to be careful of is that you give the client a receipt that discloses the additional fee (to be legal). The typical one the credit card processor sends just gives a total. We used knowledgesync to trigger a receipt with all the details.