I am considering to send some samples overseas in Eppendorf cups. They are the standard plastic cups of 1 to 2 mL capacity. Of course they may tip over during their journey and I'd like to know how well they seal or if there is any danger that anything leaks from inside the cup. The samples are fixed in alcohol if that is relevant.
I have never had problem with tubes leaking. However, if you are worried about them getting somehow popped open in transit, you might consider wrapping the lid in parafilm. Alternately, there are lid locks that you could use like these.
@Chris has a good point about screw-cap tubes, they could also work, and are probably a lot sturdier, in case of actual damage.