branchanActually all the stores get the 20% discount on shipping from Japan Post. Only HLJ passes along the discount to the customers.
Even if that's true, consider that they're not charging you for all the packaging material they're using. Maybe they use the money they save to buy shipping boxes and stuff. Who knows.
Just because they're not passing any discounts they may receive onto customers though doesn't mean that they're overcharging you for shipping so I still don't think it's fair to say that their shipping is "expensive."