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.
This case will have a lot of resources thrown at it, which means tracking down where the USB key was bought from, where the envelopes are from, printing marks, where it was mailed from, how it was mailed, surveillance cameras within distance, toll booths, traffic cameras, witness statements, door knocking in the areas, electronic purchasing records, etc. you name it.
It provides 8, 10, 12+ more bits of information in narrowing down the suspect
He sent two letters, so you double up the amount of information and then can narrow down the bits by overlapping what you find
The FBI uses a computer program now that can take all these bits of information and consolidate them, so it no longer even requires a single investigator to have the complete picture.
You can send out dozens of agents and local police to collect as much information as possible and to log it, software will do the job of spotting patterns, narrowing down ranges, etc.
By venturing outside of the realm of the internet where the advantage is with the hacker/perp you step right into a world of real evidence where the FBI has a huge advantage with resources, knowledge, information, ability to subpoena, physical evidence etc.
Very different to just a single IP address on the other side of the world
Many large and high profile cases have been broken with such evidence, for eg. BTK killer
Seemed a bit strange to think of using bitcoins but to then undo that work by leaving an evidence trail and a lot of information to narrow you down with
[edited for tone/less brevity]