Another thing is that shipping integrators use the USPS coding as well as shipping info from zencart. If the two dont match, then a new can of worms is opened. Rules & filters are based upon USPS specific classes.

My guess is that zencart is an extremely small piece of the USPS puzzle.

That being said, if you want a shorter line, change United States Postal Service to USPS and turn off the delivery times which are next to useless.