In my experience, it is completely arbitrary. I used to purchased many Burmester, Ayon and MBL preamps/dacs/amps from Europe (England / Netherlands / France / Germany). Rarely got taxed, regardless of USPS or UPS.
But for "inexpensive" stuff like Kharma low end cables, or some DIY $300 amp, USPS and UPS happily tagged on the import duties.
Funny thing is I never got taxed on stuff from Japan, Hong Kong nor Taiwan. However, 100% of the time, I got taxed on stuff from South Korea.
The rate is reasonable nevertheless, but be weary of using UPS or FedEx since they tagged on ’custom processing fee’ which is high.