I would imagine that the rate of pay for a carrier is similar to the rate of pay for a meter reader or someone else that has to regularly go door-to-door. Likewise, I would imagine that the rate of pay for the clerks is more or less about what you'd expect someone working a high-volume dock to get paid.
I'd say it's not the salary so much as it's the bureaucracy. The carriers' union has USPS by the balls.
Breakdown of USPS pecking order:
Temp - temporary - starts at $8/hr. usually no schedule, works per diem
Casual - temporary - starts at $10/hr. no guaranteed schedule, works in 3mo intervals
Part Time, Flexible ("PTF") - career - starts at $14/hr. no guaranteed schedule.
Part Time, Regular ("Regular") - career - starts at $16/hr. no guaranteed schedule.
Full-Time, Regular ("FT" or "Full-Time") - career - starts at $18/hr. guaranteed schedule.
That was back in 2007, and that's for the clerks, the least paid employees at USPS (yes, even the janitors make more). Career positions get benefits packages as well, and they are your union merabers. Moving up in the hierarchy is based solely on seniority, never merit.
So the bureaucracy gets interesting when you're distributing hours. The unions, particularly the carriers' union, have made it so that Temp gets less hours than Casual, Casual less than PTF, etcetera, all the way up the ladder. FT employees are guaranteed 40 hours per week. So the guys that get paid the most per hour are also the ones that get the most amount of hours. If you're a Casual that's working his ass off, you still won't get more hours than any of the career employees who, after benefits, are making nearly, if not more than, twice as much as you. So labor costs go up.
If one route gets overloaded, you gotta have a career employee pick up the slack if your Casuals have already used up their allotted hours for that pay period, and since that career employee didn't get to his position based on merit, it's possible that he's a lazy asshole that's going to take forever.