Best practices for dealing with zip codes and business rules

I have a lengthy form that I have broken into several steps in a
'wizard' style format.

This form requires many addresses (4-5). There is a database requirement
that cities and states must match the zip code as provided by the USPS.
There are cities that share zip codes. 

I need an accessible method of seeing to this requirement is met. My
thought is to ask for city state and zip and bonk at the user if the
city does not jive with what the post office is telling me is valid,
then giving the user a dropdown of the possible cities. 

The application owners are suggesting filling City and State textboxes
onBlur off of the zip code. So the order would be Zip, City, State, and
City and State would only appear to be editable. Dropdowns would appear
when applicable.

Does anyone have any examples of a best practice for this situation?

Thank you,

Greg Herzhauser
New York State Insurance Fund
[email protected]
518.437.5207




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