Air Post

Above the criss-crossing grid of rooftop power-shafts, a thin tower stood tall above the Capital. It didn't quite fit the definition of a 'building', as it was merely a box array of iron supports without walls, and its iron and wood staircase provided little in the way of comfort in the face of the wind. Tarvo had a hand on the railing and the other gripped tightly on the tower's outer supports as he wound his way up the staircase. The tower wasn't the tallest thing in the capital (the gondola network had that claim), but it was still a bit too high for his comfort. He stepped onto the landing with a grunt of relief, before his eyes saw the problem.
"Oh hells." The bird that had flown into the pulleys was very clearly dead, with its organs strewn about the gears and its head jamming one of the connectors. One of the mailbags that was about to get roped out had instead pinned itself between a gear and driveshaft, threatening to burst and dump mail across a city block. Tarvo gingerly tugged at it and breathed a quick sigh of relief as it shifted clear.
The bird was a different story. The morning post was already late and they had to just clear the stoppage to go, but all of the machine work needed to be rigorously inspected at some point in the evening and any broken pieces replaced. Tarvo pulled a small knife from his toolbelt and started to work away at the jam, small bits of loose viscera falling away and floating down to the city beneath him. Well, as long as none of it landed in anyone's food...
There. He gave the gearing a quick tap and the cables shifted freely, though the blood was sticky on the gear teeth. Good to go. Tarvo turned around and gingerly made his way back down the staircase, step by careful step, but this time with the whole of the Capital and the Grand Canal spread out underneath, crowds of people and power-wagons packed in the streets, with the near hypnotic motion of the power delivery shafts keeping the whole of the city in good cheer. 
Sidonie was waiting impatiently at the bottom of the stair. "What was the issue?"
"Bird strike. Bad one, jammed up the number five conveyor." Tarvo pulled off his bloodstained gloves and tossed them aside. "Should be all good now."
"All good! Start the belts!" Sidonie yelled out, and the great gears of Central Post began to turn, and the mail was rising again.

Utility

A massive tower was erected on top of the central Post station in the Capital, with a number of conveyor chains ascending to its top, corresponding to different destinations. Bags of mail are attached to each chain with a specially designed locking clip, quickly rising to the top of the tower. At the top of the tower, the clip hooks onto a guy wire, pulling the mailbag free of the conveyor chain and causing the mailbag to descend down the guy wire to its destination at a rapid pace. This allows for rapid distribution of mail from any Post station with an Air Post tower.
Air Post has significantly reduced mail delivery time each morning, with most overnight letter type mail being fully distributed by midmorning. This has also taken a large number of Post service Power-Wagons off the streets in the morning traffic. Capital residents have become accustomed to the sight of bags of mail zipping by overhead.
The system is not without flaws, however, as the mechanisms are prone to breakdown. More than once, mailbags have been lost in midair and strewn their contents across a section of the Capital City. Defenders note that this is not particularly different from a Post power-wagon being involved in a traffic collision, or someone ramming a Post delivery box.

Manufacturing

Principality of Etoile construction teams built each tower, but the plans are freely available and any private individual can theoretically construct their own Air Post towers with market-standard Power components. No privately owned towers have yet been approved in the Etoile Capital City.

Access & Availability

Eleven Post Stations have Air Post towers built, in a rough ring around the Central Post station. Mail is sent from the central Post to a district station, and if the district station has its own Post tower, mail is sent on in turn to a smaller location, such as a neighborhood substation or some particularly high traffic mail recipient. Anyone can send mail this way, though all mail that's collected from senders is typically carried by hand in the traditional manner to the post office.
Deployment of Air Post in other major cities is pending, as the system is only recently established in the Capital City itself. Smaller towns and villages are likely to continue traditional postal delivery, and the required heights for distance make the system impractical for inter-city postal delivery.

Complexity

The technology is not particularly complex and uses widely available parts, though repairs are frequently necessary. The towers themselves are architectural feats, high spires built to minimize weight and cost, though none look forward to ascending them.

Discovery

Air Post is a relatively new way to deliver mail, exclusive to the Etoile Capital City. A Power engineer task group was charged with developing a way to apply Power technology to the prosaic task of delivering post, and they came up with this concept in 721. Trials were promising, and full construction and deployment was completed by the end of the Third Season in 723.