Why on earth would you tell them they had to pay within 3 days when you know ebay allow them 8! Why would you send them anything after the auction finished?

Had you not done that they would probably have paid within 4 or 5 days and left you pos feedback once they received the item.

When dealing with buyers customer service should be at the front of your mind at all times, of course as you have a merchant credit facility you must have another business so should be aware of that anyway. I don't let buyers cost me money but I stay professional and polite in all dealings with them and never make unenforceable demands as it just makes you look silly.

If I were them I would probably pay on day 7 and then the neg would stay!