I've actually discovered that only prompting for zip code and not city/state can be somewhat confusing for users. We have a system that only requires 3 pieces

I've actually discovered that only prompting for zip code and not city/state can be somewhat confusing for users. We have a system that only requires 3 pieces of info to normalize your address, including city/state: Your street address, your apartment/unit/suite number if applicable, and your zip code. That gets submitted into a third party system that, among other things, normalizes the address to the USPS standard. (For the record, this is for an application that only deals with US customers, so internationalized addresses aren't an issue here.) We've typically only prompted on our localization pages for those three pieces of data, and it's surprisingly confusing to customers. It's not too unusual when we review the error logs to discover that people attempted to enter "19000 Euclid Ave, Cleveland, OH" into the street address field and "44112" into the zip code field. Unfortunately, the third party system we use doesn't like this, and will generally not return useful results in that case. It's getting to the point I'm considering either adding city/state fields to the form or just a message saying "city/state not required." I'm actually not sure which is the better approach.