The same blisters were used for the M Diancie packs, and those weren't expensive to have shipped. I ordered one from AmiAmi, paid my ¥380 for the weight class, and it arrived in a flat box with padding in perfect condition. Of course, being a large company I wouldn't be surprised if they got some kind of volume discount from JP Post that helps mitigate these costs, which wouldn't be available to smaller resellers.
I know I could just buy singles, but I prefer to buy sealed product rather than singles, even if it's just promo stuff with extra cards like these. I try to avoid buying singles wherever possible.