I will never ship international again!!

OK all I can say is eBay need to set a rule regarding age limits of buyers. This buyer I am very upset with lives in U.K. I made a mistake w/ customs papers. A very simple mistake. She has not left me negative feedback, my first negative score. She is impatient and immature. I feel like I am dealing with a child. The Feedback she left was false & very rude! I can't do anything abt. feedback she left since I was so kind to leave her with Positive feedback. I am very upset! I wish there was something I could do about this! Oh well. I think I am done selling on eBay. Some of the rules are stupid!