Nobody is holding a gun to your head and forcing you to use ebay. Instead of refunding postage if you had just reinvoiced with the correct amount before they paid you would not have been charged on the higher amount, why should ebay be concerned if you stuff up and quote the wrong postage. If you had undercharged and the buyer chose to pay the extra would you have contacted ebay and offered them the extra fvf you would have been charged had you had the right amount on the invoice?