> What's the 5 digits for Yonkers New York (edited because I originally had NYC)

Nobody sends packages where the destination is an entire city. If someone gives me an address inside Yonkers, it'll have the zip code in the address. I've never had to look up a zip code in my life.

> For the same reasons, that's why it would be: Beverly Hills, Los Angelos County, California, USA, 90210

Which reasons? That has nothing to do with magic numbers, except that a 'magic full mailing address' is still bad, you don't shove that into the middle of your code either. If you're looking at the "made a typo" reason then that's where showing the address after putting in the zip code will give you the same verification but faster.