I'd be saying something like:
"No problem. To pay the remaining amount, which do you prefer - bank transfer or PayPal payment?" Don't even seem to give the notion of accepting less than full payment your consideration. Act as though you're assuming they thought all along that they'd be paying the extra by either of those other means.
If the buyer pushes it, mentioning that it's just a small amount, I'd be saying, "I'm charged on the amount for which you won the item, so I can't give an additional discount, unfortunately. Of course it is a small amount, so bank transfer would be the easiest for you as it attracts no charges. You can log into your internet banking from here and make the payment, and then the transaction will be complete. Sound good?"
If the buyer were stubborn, I'd say, "I'm sorry, but we do have an agreed-upon price. Margins can be very narrow, so please don't be offended by my refusing to give a discount."
If super-stubbornness eventuated, I'd say, "I'm going to need you to pay the full agreed-upon price; you can either pay that now by whatever method is best for you, or come back later with the full amount."
I wouldn't even suggest that the buyer walk away and I'd make an Unpaid Item report. The final statement needs to be an either-or situation that is friendly but unmoving; if one offers the buyer in that situation the chance to take umbrage, they will (because the buyer will have an internal dialogue that's something like "Are you kidding me? It's just a few dollars" and will be justifying it to themselves in full Dolby Digital Stereo). Successful negotiation comes down to offering choices - and making sure that those choices are geared towards giving you the outcome that is best for you, and most likely to be acceptable to the other party.
(The Unpaid Item report is your personal recourse if the negotiation fails.)
