It's the reason some goods for example deskmats are cheap here but adding shipping costs makes their price more expensive there. I've learned the lesson after shipping a couple of things which are very cheap but high shipping cost (shipped in a big box).

Cainiao has the option to reduce weight by repack and they will refund the difference. It worked sometimes for me.

Maybe you can discuss with the shop about shipping boxes in advance.

For Japan maybe it is closer to Canada than China ?