While theft is certainly likely, the possibility of negligence on the part of the shipping agency is also very high, especially if mailed via the USPS, as a rule, I NEVER make any deal over $1,000 unless I can meet in person. This type of thing happens far too frequently to trust such a precious item to the mail, be it incompetence on the part of a postal worker, or the actions of a pathetic thief, it's just too valuable.
I REALLY hope you are able to recover it! And that the only regret is that if it is a thief and he caught, that he be merely locked up rather than something more...fitting.