Don't ship valuables
Hi,
When I moved from Japan to Europe (Austria), I sent my things via the post office (surface mail). Everything arrived intact and all in one piece. My friend did the same thing a year later and every thing also arrived intact. One thing though - the authorities opened the box and patched it back badly. So be sure to use a sturdy box.
If you are worried about things getting wet, pack them in those big plastic boxes with plastic lids and wrap them with brown paper. This was what I did. My things did not get wet but the box did crack a bit from the rough handling.
My other friend shipped her things by putting them in brown boxes but everything was ok and did not get get.
When my other friend moved out of Japan, she choose to ship her things with a shipping company who is supposingly reliable and so she paid alot of money to ship through them. However, everything arrived but her expensive box of kimonos (Only got minimal compensation from insurance agent, not even enough to buy one kimono).
So, I don't think it really matters whether you ship your things via the post office or a shipping company. I believe they will arrive intact except when your things get stolen.
All the best!