[TMWNN]

I admit to being inexact, but my larger point remains: Shippo's analysis (and most of the anecdotal reports about "slow mail") is regarding "parcels". (If you're going to be sneeringly pedantic, the USPS term is "parcels" and not, as you claim, "packages"; the word appears in USPS descriptions of levels of service, like "flat rate package" and "first class package", but is not a category like "letters", "flats", and "parcels").

To put another way, "flats" are larger "letters" and can be processed in similar ways, because both are bendable, and thus machine-sortable in a way that "parcels" are not. This is also why letters, regardless of height/weight, that exceed a certain thickness automatically become parcels, because they are no longer bendable.