From having previously scoured this forum: keep the liquid under 200ml total and include a few other bits (like cartos etc) in the package. try to get the vendor to label it as anything other than
eliquid.
I don't think mods will have any problems.
i believe juice with nic is 600 won tax per ml (about $16 on a 30 ml bottle). If you are really concerned you may want to buy nic base and nic free juice and mix it yourself. that way most of the liquid would be nic-free/tax-exempt. Another way would be to buy half your juice in 0% nic & half in double your desired strength and mix them together.
FWIW: I've had 5 orders (from China not the US but i don't see why it would be different) all with 100-200 ml liquid and other bits and none has been stopped.