I've asked if a trace as suggested can be done and the answer was negative.
The only way was to send a complaint form asking refund for undelivered items, lost item etc. that would be turned to foreing post that start the search.
I tought that sending it after only 10 days would have been a bit hasty, so I waited a bit considering that usually usps is pretty slow and christmas backlog.
I've shipped two EMS the same days, one was delivered (not arrived) to Japan 14/12, the...