Oh sorry, nobody got around to answering your question yet! Bookwalker vs EbookJapan is just a matter of (a) my personal preference and (b) based on some discussions in the forums, also the preference of quite a number of other readers who are forum members. And so we sort of informally agreed somewhere (I think?) to list Amazon and Bookwalker in the book clubs to give some examples of where to find the books, but we don’t try to list each and every option where you can buy the books. For the price, each of the book dealers run sales every so often; sometimes they are the same on different platforms, sometimes not. So it’s not a general rule of it being cheaper.
Here are some general information threads:
and if you generally search for the shop you’re interested in, you will find lots of information on the forums.