how to ship shock absorbers from japan?

[robotype]

for the first time in my life I can't bring something out from japan...

while I was there for christimas I got a pair of used coilovers and rear shocks from a friend. bubblewrapped them and threw them in a suitcase, brought it to the airport and checked it in, no problems so far.

boarding time: they call my name and say they can't take my suitcase on board. they x-rayed it and saw the suspensions and since there might be gas inside they could be dangerous etc.

so I asked the flight assistant to send the suitcase back to my friend and then told him to send them over to me by ems. same story. they x-ray the box and you can't send it.

so now I am thinking to send them by sea...
experiences? ideas?