Should be titled "Put the Zipcode and country first".
Would be simple enough to have someone type the zip or postal code first, then move the countries that are known to have that zip code to the top of the drop down (still in alphabetical order), and all the other countries below that (also in alphabetical order).
Leave all the fields editable, i.e. the postal code only suggests, ranks, or prepopulates.
Seems like that would handle all the edge cases, including those where a zip code doesn't refer to one and only one city, or where a zip code has changed or been added to a country.
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.
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”.
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 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.
Would be simple enough to have someone type the zip or postal code first, then move the countries that are known to have that zip code to the top of the drop down (still in alphabetical order), and all the other countries below that (also in alphabetical order).
Leave all the fields editable, i.e. the postal code only suggests, ranks, or prepopulates.
Seems like that would handle all the edge cases, including those where a zip code doesn't refer to one and only one city, or where a zip code has changed or been added to a country.