As a (non US) buyer I am willing to pay reasonable handling charges (say a couple of dollars) because there is slightly more trouble involved in international bids. Also I sometimes have to ask sellers to "oblige" me by stating a lesser value to the widgit so I don't get whacked with import tax. In my experience many sellers do not know how to ship internationally (I mainly buy small amounts of textiles which can go as USPS global priority) so I end up telling the sellers how much its going to be and query (you can check up on the USPS site) it if I get a high estimate from them. So I don;t mind paying a reasonable amount over the top.
As a SELLER It's difficult to be exact as the things I sell vary in weight/size but I am able to give a "ballpark" figure in my TOS and a proper estimate to the high bidder at close of sale. I charge reasonable S/H (about $2) to cover my trips to the post office and the time taken in packing the items. No-one has ever complained about this.