If there's any good to come out of this, it's that many will learn a valuable lesson in best practices when shipping, and not just international. IMO, if international is done properly, as many have described here, these kinds of situations can be completely avoided (not to say packages won't occasionally get lost).
Anyway, on to constructive input. Had a similar situation: First-Class, uninsured package to Serbia not delivered after seven weeks. The differences: I hand-delivered to post office (invaluable advice given by others here in that regard). That ensured the customs form was scanned and printed on my receipt. Unless I'm a crafty thief, having a receipt containing date, weight, destination country and customs number is de facto evidence that I did indeed ship the package. Quick aside: for international shipments, I always take photo of package, with address label showing, and provide to buyer. Doesn't prove anything, but along with the aforementioned receipt, provides buyer with a little more peace of mind that being handled properly.
With the presumably lost shipment to Serbia, I'm feeling badly even though did everything right. I input customs number online as starting point. It's showing "no record." WTF!? I go to post office where I shipped, with customs form and receipt in hand. Clerk tells me after 45-days, the records are taken off-line ... archived. So, what can I do now I ask. She gives me toll-free number (1-800-222-1811) to investigate further. I call. At first, a recorded message instructs me to input customs number. Expectedly, tells me no record. Punch the keys to get a live person. Helpful lady. Says can have the record restored for online viewing (takes a couple of hours to implement). Three hours later, using customs number, I'm able to view tracking from Phoenix to Los Angeles (who knows why it went west, when destination was Europe). Interesting aside: according to the online record, it took two weeks for the package to travel Phoenix-to-Los Angeles. And strangely, the package was delivered in Serbia a few days after my online query using the customs number. Hmmm.
My point ... if the customs number is accurate (after correcting that last alpha-character to "S"), the record can be restored for viewing. Might not help to track down the knife, but it will answer that one fundamental question: Was the knife actually shipped to Russia?