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Author of the blog post here.

Access to the API isn't automatic. You have to apply, which I did, and then was turned down. They only grant access if you're using the info to ship packages via the USPS. I was completely honest that I wanted the data to build open-government apps of different kinds, and they declined me.

Looks like USPS API data is free and usable only for package shipping. Not that I bet they would notice. At any rate, this seems to be a great start with a great API recommendation: http://stackoverflow.com/questions/7129313/zip-code-lookup-a...

Sorry to keep replying, I don't have an edit button for some reason. This one is cool too: http://api.zippopotam.us/us/ma/belmont

I found a town called Bakersfield that has more than one zip fro the town, it does JSON back with more data than I thought would be part of free plan.

For some reason I could not get "san_francisco","san-francisco", "sanfrancisco", or "sf" to work.

http://api.zippopotam.us/us/ca/san%20francisco

if you take a closer look you'll notice that the usps api takes a USERID as an xml parameter so it is not free.