Also, there are a few zip codes that cross state boundaries. Those rare cases involve very rural areas where the nearest post office is across the state line.

The +4 bit of the zip code narrows the location down to a block or street. In the case of apartment/condo buildings, each bank of mailboxes (for each lock that the mail carrier uses) gets their very own +4.

Sample picture: https://postalsupply.net/2023/03/maintenance-and-upkeep-tips...

This gave us such a headache 20 years ago in earlier ecommerce days. One of many edge cases. I'm in Colorado so we always tested with 82082 which covers areas in WY, NE, and CO.