What can be done if you have posted out an item in perfect condition and the buyer claims otherwise?

[sethari]

What can be done if you have posted out an item in perfect condition and after a little while, the buyer claims the item was delivered in a broken condition that is:

1 - the fault of the post office, but ultimately YOUR problem because they are expecting you to do something about it. Postal insurance cannot be claimed because the required packaging has since been disposed of by the buyer.

2 - your fault. When they opened the intact/undisturbed packaging, their item was broken and smashed, etc.

I mean when it comes to the crunch, what can be done? If they're demanding a refund due to the 2nd problem and a seller refuses out of absolute, (yet unprovable) certainty that the buyer is lying, is the buyer justified in leaving negative feedback?

Please help!!!