Paying Tax for shipping my own possession?

I’ve shipped my unassembled pc parts here and I I just got a call that told me to pay about 10200¥ for tax,

I wonder why I should pay shit for my own possession?

The stuffs are obviously used…

It costed me LOTS to ship the stuff here and now another spend…🤦🏼

Please let me know if I misunderstood something…

—- update

Thanks for the reply, I asked my family to ship my unassembled pc parts from Taiwan to Japan, and I thought I should BE HONEST TO Write down what “EXACTLY IN THE BOX” in the declaration!

So I asked my family to write “computer parts”

DID I DO ANYTHING WRONG? Seriously, or should I kinda LIE and say that is my unaccompanied baggage.