Sunyshore charges more than MSRP as their commission fee for spending the time and effort to obtain these goods. Limited time, PokéCenter-only TCG products especially (like this one!) are a pain to get because the stores are very crowded and busy, like shopping in a popular store in the US on Black Friday. Because the shop is run by fans who resell product to other fans (rather than being a large corporate entity like AmiAmi that receives wholesaler discounts), these fees are the only way they actually make money from sales, and therefore how they can afford to stay in business and offer us these cool products.
Honestly, Sunyshore's mark-ups are pretty reasonable, especially when you compare to some of the ebay listings up for the same product. I'm seeing 2 different listings available now, one for US$86 and one for US$62+shipping. PokéVault, another popular Pokémon reseller website, is selling the full boxes for US$85+shipping, or as individual pieces for the various contents.
If you'd like to see some other options for purchasing, I would suggest you check out the
Getting Stuff From Japan guide in the collector's forum that I wrote up with the help of SR. One of the section contains links to various Middleman services, which are basically website companies that allow you to shop and bid for items from Japan-only sites, such as Yahoo!Japan and Amazon Japan. Keep in mind that services do also charge their own commission fees for the work that they do, though you may still be able to find a better deal through one of them if you work at it.
I ordered my box from Sunyshore today.
I'll probably still be on vacation when it arrives, but it will be something nice for me to look forward to when I come home.