For future reference, always always ALWAYS have your local USPS scan in anything that has a tracking number/barcode. If you do not, it is not guaranteed that they will have it scanned at the origin point (which appears to be what happened in your case). If you do not get your item scanned at dropoff, you cannot prove that you ever dropped it off. If it gets lost before it's scanned (usually happens at the first distribution center) you are SOL even if you have insurance.
Had you taken your letter to the counter and had it scanned, it would have shown that in your tracking info. Only once have I ever had a tracked package not register an initial scan, and that was with a Fedex Smartpost package at my local USPS (supposed to only be dropped off at Fedex, but USPS will take them).
Dropping packages in the package chute or leaving them on the counter is really convenient, but it's not worth the headache when things go wrong. USPS is pretty good, but they do screw up more than the other carriers--and the risk of a loss is much higher when shipping internationally.