I send the ones in the plastic cups -- no special packing. I haven't heard of them breaking open -- yet ;o). If I'm enclosing something I don't want to get wet, I put that in a ziploc bag. And I really cram the stuff in so it doesn't rattle back and forth in the box.
It's better to send fruit in the cans, with pop-top lids. The plastic cups are very prone to breakage. Think of this: I sent a well packed box, all chunky soup solid cans; no shifting of contents was possible. It was returned (because the post office mis-read my directions on the customs form). Every single can was dented. The box was also full of the talc-powder-like dust even though every opening and seam had been sealed with dust. So if solid metal cans get dented, expect the plastic cups to break.
Anything liquid should be in DOUBLE zip-locked baggies.