How the US Postal Service reads terrible handwriting
youtube.com
youtube.com
Also, in the video they show a poster about keying in letters to Santa. You can actually sign up to fulfill the Christmas wishes of random children:
The keyboard layout was custom for the application like they show in the video. The software would expect numbers at certain times, so you'd just type numbers using the home row and it'd switch back over the alpha chars after you input the correct number of number chars.
If you pause at 4:01 you’ll see a real time model forecasting the number of keyers online within the next few hours. I assume that’s done because everyone still essentially works in that center, though I might be wrong.
I meant all sorting depots, not all mail.
Presumably each depot has manual systems, but those systems will be very inefficient and cause backlogs and other subsequent problems. A manual system for a large depot that suddenly gets 20x or more overloaded would struggle with an extended outage of that one centralised system.
Finely tuned flow systems often break down spectacularly with unexpected disturbances: look at the problems at container ports. Or play factorio.