It's easier said than done. How do you politely tell a customer you don't want to work with them, without escalating the situation? I for one haven't managed to figure it out. No matter how bad a customer might seem, you have no idea how insane they can get if you refuse them service. This "20%" is like nitroglycerin all they need is the slightest nudge to go berserk!
LOL,did you even read ?just refund and apologize and move on. make sure you report that buyer to ebay and other sites as well