The ebay guidlines are quite clear regarding excess postage & packing fees and show good examples.
Difficulties however can sometimes arise, particularly when a bidder / buyer falls outside the region/s to which a seller states he/she will post.
More often than not, the potential bidder / buyer will send a message to the seller asking if they are prepared to ship to a certain region / country.
Sellers beware, because at this point you are entering into a grey area. If you agree to ship, the buyer will very often log on to the Royal Mail website and refer you to various listed cost tables.
They tend not to take into account your personal costs, ie, cartons, bubble wrap, tape etc, nor do they consider that it may be more involved (for example filling in customs forms etc) and therefore impacts upon your time. It can take longer to pack, use more materials, involve filling in more forms than for standard despatches to the UK and subsequently cost more than the actual cost of postage.
You need to make very clear in your response that the costs will include materials and labour or you could end up with heated exchanges or worse still lose money on postage. You are still left with the difficulty of persuading the buyer that your costs are reasonable, one would ordinarily assume that this would be agreed prior to the bidder placing a bid. This is not always the case and I don't find any ebay rules on this situation.