Inferring the City and State from a Zip Code

I imagine this is probably 50% standardization and 50% "preventing typos", but a lot of e-commerce sites will have users enter their City, then their State (often from a dropdown box) followed by a Zip Code. The previous two entries could have been inferred from the Zip Code alone though (via the USPS API, for example).

Would it be bad design to ask for the Zip Code first, then perform an API call and fill out the City and State appropriately? This would give the user the option to review the results and correct any potential errors, and would make for less typing during registration or checkout.

There might be some huge issue I'm overlooking with this approach though, so I'd love to hear your opinion on it. Thank you!

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