Good day Kantaro, Just read your post as l was going to message on a similar subject.
I have experienced various problems from Customs, 2 which are current. But to reply to your post, antiques have exceptions, which means they are less stringent than for modern items. Items abroad are required to have commodity codes, and antiques have 2 depending on their approximate age, over 100 years or over 250 years (clarify with the requirements relating to the country you are importing from). Commodity codes are important as antiques attract a %5 tax compared to %20 for newer goods. The sender needs to include an invoice detailing what each individual item is made for, made of and approximate date with commodity codes and whether there is a requirement for additional certificates or restrictions.
I believe many custom departments are inhouse within the relevant parcel courier and may not be aware of the regulations.
So in my case l have had a DHL parcel containing kozuka from Denmark returned to Sender because it was deemed it required an over 21 signature. Noone contacted me before returning the parcel. I had to explain that they were handles only, no blades and l am (well over ) 21.
A tsuba parcel I'm still waiting on from Australia apparently was sent without an invoice. Parcel Force required an invoice, description, dates and commodity codes. If the issue isn't fully rectified it will be returned within 28 days of Parcel Force receiving it.
Another DHL delivery from a Swiss auction house was incorrectly charged at %20 VAT even with an invoice, made worst by DHL quoting it was an approximate charge. I am awaiting confirmation from either DHL or Customs.
I have also had Customs queries regarding the value of orders, one from Denmark was sent back to seller who clarified the value, one from Japan was clarified before it could be sent back.
I also had a tanto confiscated 7 years ago because Customs didn't know it was an antique. I got compensated for that one. I spoke to an understanding representative who told me that Customs has been taken over by private companies staffed by inexperienced people.
Hopefully your issue will be sorted soon if it hasn't already.
Neil Bolden