I used to do this when I worked for the post office (not in the US) and it was by far the dullest thing I have ever done. Truly loathsome work. It was referred to as "coding".

There were rules in place so you could only do it for 20 minutes at a stretch and only a couple of times (three?) per shift. Presumably this had to do with the fact that the quality - which was a problem - would collapse otherwise. You needed to keep a low average time per letter or you would get booted off coding duties. I can't remember exactly how long, but you needed to be able to code a letter in a matter of seconds.

The American setup looks a lot more hardcore (six screens?!) than the one I used though. Pretty sure the application was written in Visual Basic.

The machines which sort mail are pretty bad-ass. Not only can they sort by postcode, but also by address so the mail comes out in the order in which the mailman visits each building on his route.