They use the online postage calculator which is notoriously inaccurate. When the inaccuracy is pointed out to them and you ask that they check with the post office, many say that's it, like it or lump it. Thankfully I've struck sellers that are prepared to double check, and once they realise the huge discrepancy they've nearly all given me the proper rate without argument.
You have to challenge obviously exorbitant shipping fees.
