Japan Post is the cheapest option but they have a maximum dimension Height max size is 1.5m Width x 2 length x 2 total must be less than or 2 metres. EMS is the priority air mail 30kg max.
But obviously your bodykit is way bigger so only way is by those moving company which charges a lot. Try calling up car import companies and ask about their shipping service if they can put your bodykit in one of their containers.
I myself is having a hard time about my stuff here in japan, wish I can just take it with me in the plane.