ZipLookup.zip
A common shortfall with many examples of handling this effort have to do with zipcodes that cover multiple cities. This example will let you easily define any number of cities residing in the same zipcode.
Here's what happens when you type a zipcode that's not in the DB:
And here's what happens if the zipcode you type in is already defined in multiple cities:
If the above popup doesn't list the desired city for that zipcode, you use the ZipCode Maint button to add it.