> how do you then even know what ZIP code to use

There has always (since the late 90s) been a free look up webpage:

https://tools.usps.com/zip-code-lookup.htm

The data used to be available on CDROM (back in the 90s) but moved to an API in the early 2000s. Until August 2026, you used to be able to look up addresses for free.

To get the biggest discount on postage (one place I worked at mailed about 3k envelopes per month) you have ZIP+4, to pre-sort the mail and print a bunch of stuff on that barcode [0]. Next time you get an envelope, look at the barcode on the bottom.

https://en.wikipedia.org/wiki/Intelligent_Mail_barcode

0 - doing it that way saved about $450/month versus not doing any of their work for them.

Zip codes do not have clear boundary lines. A zip code is a collection of postal routes, and their borders tend to move about.

This isn't the USPS charging for existing data. They created a new data set that's only relevant to data brokers, and are charging for it.

Iirc when I worked with these in the past ZIP codes did not map cleanly to boundaries, hence the ZCTAs. Zip codes are sometimes assigned to points, lines, they can overlap, and other weird arcana because they defined mail routes rather than tesselating areas.