Yeah. But also, are you ready to maintain a postcode to city mapping for every country you have a significant userbase in?

I'm definitely not, but the lazy method is just to cache the data your customers provide, or curate some data for the countries you are most interested in.

That gets around countries that consider their postal code database a licensed dataset.

That’s a clever way of doing it, yeah.

From UI side, I’d probably not autofill, but add autocomplete (e.g. using <datalist>¹). This way you can still have browser autofill the user’s address if saved (do also set the autocomplete² props correctly!), and suggest one or multiple variants you have in your database, and let the user fill it out manually still, without having to find and click “My address isn’t here”.

¹<datalist>: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...

(quick note re: compat; Firefox does support it for the text fields, check the table at the bottom)

²autocomplete: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...

ZIP Code (tm) is claimed as a trademark by the US Postal Service. If you don’t have a ZIP Code, why would you put something in the ZIP Code field?

Because most websites won't have two separate fields for "ZIP code" and "postal code". Even if they knew that ZIP Code is a trademark (I didn't until you mentioned it), they would (wisely) know that putting in two fields would just confuse most people. So they put in one field. Many sites label it as "ZIP code / postal code", but some just label it as "ZIP code". But the intent is clear: put in the multi-digit string that identifies your address, whether your country calls it a ZIP code or a postal code.

Put a postal code first.

Include a field for the country that is autofilled via Geolocation API or geoip if that's unavailable.

Put country first. Postcode doesn’t make sense without one. Prepopulating one based on location is generally okay, but don’t assume it will always be the same.

Or just don’t try to be too clever.