If you can't get the neutral removed, respond to it saying something like, "As you said, the problem was caused by Aust Post", just to reinforce what they said. I seriously doubt it'll stop people buying from you. I've had the odd negative from buyers who don't read listings properly or look closely at the pictures - all it did was increase my sales because buyers felt sorry for me. You'll probably notice that some of your buyers will go out of their way to leave extra nice feedback to help counteract the neutral.

You have to learn to live with the odd negative on ebay. Life isn't fair and ebay is no different. You just can't expect to get justice 100% of the time so you have to learn to cope when you don't.