EBAY FEEDBACK....WHAT CAN I DO?

I've been on ebay since 2001 when I brought a computer home for the kids for christmas,my oldest daughter showed me how to run it,Since then I joined ebay,buying things like crazy,recently I started selling,due to the economy and working construction being laid off in the winter months,finding items laying around the house that i dont need any more,many from auctions which was my weekend hobby, till my favorite auctioneer died,and things I bought on ebay,which I didnt really need,so through out the years I have maintained a good relationship with people,both buying and selling,recently on ebay I had a box of beer handles,beer keg type, and put them out there for .99 cents. Wow to my surprise they were selling like hotcakes.I couldnt keep up to the emails,Questions, and the hussle and bussle that came at once,now I have to weigh them ,figure shipping prices,get them to the post office,send out emails,shipping labels answer some more questions, wow thats alot of work for 99 cents.After a while i tried to stay on top of it.Trying to give people the best shipping price for their money.To make a long story short I sold a'll but one out of 55.But after its all over with theres feedback coming in.To my surprise I had one buyer out of 54 which left me negative feedback ? on three items .99 cent items,why?? because i sent an invoice once with the shipping charges stated.must pay within two days of auction ending.2 days go buy send another invoice,no replies or nothing,send an email asking qoute(hey ,are you getting my invoices?) Finally get paid and ship out within 2 hours...Now I'm dealing with this person to get my reputation back as it was.Almost 5 stars across the board.....(.THE NEGATIVE FEEDBACK STATED I WAS RUDE.)What Can I Do...I cant sleep at night...!!! I could tell you the persons name and email address and maybe you could ask them why??? I need some answers Thank you for reading this