[lukeschlather]

That's like suggesting people don't need to know what the zip code is because it's often redundant and omitted. People are often lazy, but it's immediately obvious to anyone that omitting the full 9-digit zip code could result in the letter being misdelivered, even if I don't understand what the last 4 digits are even for.

[talldatethrow]

I'm 38, live in the SF Bay area, and have never given anyone more than the first 5 digits in my life. Online some might auto correct, but I've never learned them in my life and never even considered I should.

[tristor]

The Zip+4 last four digits align to delivery zones. It can be trivially constructed from the complete address now that we have reliable digital mapping systems, and in fact this is what happens internally in the postal system.

It is not required and will likely never be required to provide a 9 digit ZIP for reliable delivery. It may, and does sometimes, impact speed of delivery due to sorting/distribution rounds.

[labcomputer]

> It is not required and will likely never be required to provide a 9 digit ZIP for reliable delivery.

That depends on who you are.

If you are a regular person, then yes, 5 digits is sufficient. But if you are a sender of presorted commercial bulk mail (which is discounted from first class), you may actually be required to provide a 5 + 4 + 2 = 11 digit ZIP.

That little barcode the post office prints on your letters is actually just the 11 digit zip. The final two digits are the last two digits of the house number. So "123 Any Street, Anytown FL, 45678" the final two digits of the zip would be 23.

[LoganDark]

It's honestly not that obvious. I never knew there's a difference between the 5-digit and 9-digit versions of my zip code. Most checkout flows do not even allow me to input more than 5 digits in the first place. But upon receiving my mail, the 5-digit code is always corrected to the 9-digit one.

I had never considered that if there were multiple 9-digit expansions of a 5-digit zip code, the correction might turn out wrong unless the full 9-digit code is specified.