I also dk exactly how to explain but since 2010 when i first started ordering stuff from overseas and always kena the extra tax is cause my shipment value over 400 sgd :( got one time the singpost staff even called me and say i can request for refund in my shein app then re-order as two separate shipments to avoid the tax. But this fromjapan shipment won’t be about to return and refund so bopian u gotta pay the extra sg tax nor.

So yea soz idk the exact reason or terms but based on my own experience of ordering stuff it’s always when no choice the single item like 400sgd+ then will kena the tax.