You can walk up to any mailman/mailwoman in the USA and hand them a stamped, addressed letter and they will throw it in their mailbag and then at the end of their shift they will dump all the outgoing mail into a sorting machine.
There are un-surveiled postal drops all over the USA.
Remember the anthrax letters? Case was never solved.
You are technically correct about the anthrax case, but that's because the guy who most likely did it killed himself.
I know that the original suspect was the wrong guy, and that the government basically tried to ruin the guy's life anyways, but the second suspect (the dead one) seemed to be the real deal.
This doesn't really detract from your statement about the post office though, I believe they found the anthrax guy because they can trace the source of the anthrax.
EDIT: It's been a while since I looked into this case, but apparently in the last few years a lot of the evidence has been called into question by external scientific organizations.
There are un-surveiled postal drops all over the USA.
Remember the anthrax letters? Case was never solved.