The only real difference in the handling of first-class letter mail and the handling of householders is that there's a different delivery standard.
There's a clean-floor policy, which says that the letter carrier has to take all of the first-class letter mail that's in their case out when they leave for the day. With the householder mail or advertising mail, they're given three days. They take a third of that mail each day, because of the load, the overburdening.