I don't want to throw a cat among the pidgins here but IMO the seller should be responsible for the oversight, I know it is not always easy to calculate the exact cost when sending over seas till after the auction & when you know which country you are dealing with, this is why I actually put my international shipping cost at the highest rate shown on RM for overseas for the weight I am sending, then should it work out less I simply refund the excess, it does put some buyers off, but it covers your back.
Surely if it had been a small oversight the excess charge would only have been a few pounds, not twice the cost as its not that difficult to calculate costs for over seas, I mean I did it earlier in this thread & it took me all of 2 min's to look up & come back here.
I personally think it would be a very kind gesture to offer half & would appreciate this from a buyer should it have been me making the error, but in all fairness how often does this happen?, most times you make an error - you bite the bullet & just use your mistake as a learning curve.
When I make partial refunds if I have over charged on postage 9 X's out of 10 you don't even get a bloody thank you for your troubles, though this does not stop me doing it.