[IsThisObvious]

More interesting if you could make a mechanical one that would remove one envelope and forward the letter.

Then each data facility could reasonably process a few thousand letters a day, batching them for the postal service. After a few rounds of mixing, if there was significant facility-to-facility traffic, it would become impractical to find any specific letter's path.

Time for real world implementations of our high-latency packet routing algorithms?

[nitrogen]

The next step is building such a system for personal transportation pods, so nobody really knows where you're traveling ;-). However, you'll have to pack enough food to be shuffled across the country several times on underground pneumatic tubes as your personal carrier onion is unwrapped and retransmitted.