You can't alter the international postage fee after a sale has taken place, but for future reference what you could do would be to try listing the item without an international postage option to start with, making sure that you include something in the item description along the lines of "International Buyers: Please contact me for an international postage quote before committing to purchasing the item." That way if an international buyer was giving serious consideration to the idea of buying the item then when he/she saw that statement the buyer ought to contact you to enquire about the cost of international postage, at which point you could research the cost of postage to his/her country and then get back to the buyer with a quote. If the buyer was happy to proceed with the sale for the total price, inclusive of international postage, then you could revise the listing, add in an international postage option with the price that you had agreed and send the buyer a message to state that you had added an international postage option so that he/she could go ahead and purchase the item.
Before anybody starts saying "That wouldn't work" I know it is possible to do this as I was once the buyer in such a scenario and I wanted to buy an item being sold by an American buyer, even though there was no international postage option listed on the advert, but after sending the seller a message to ask if he would be willing to sell me the item for the full asking price, plus international postage costs to get the item to the UK, the seller told me that he would get a quote for me and get back to me about it, which he subsequently did. The international postage costs quoted were lower than expected so I agreed to proceed with the sale and the seller revised the listing to include an international postage option so that I could purchase the item, which I subsequently did.
