I like the autocomplete too. Probably the best pattern, but it does usually do that by leveraging paid Google Maps APIs I believe.

I was frustrated that this seemingly open data wasn't openly available. Anything that asks for city+state+zipcode can ask for zip code first, auto populate the rest. For the edge cases where the city is wrong, the person can still type in the city like they would have needed to anyway.

It is worth noting that a package would never get delivered to the wrong place because the city wasn't correct but the ZIP code was. The USPS routes based on ZIP codes, not city/state.