Address field module autofill city and state when zip code is entered
Here is Javascript code that uses zipcodeapi.com API:

&lt;script type="text/javascript"&gt;//&lt;![CDATA[
$(function() {
    // IMPORTANT: Fill in your client key
    var clientKey = "js-9qZHzu2Flc59Eq5rx10JdKERovBlJp3TQ3ApyC4TOa3tA8U7aVRnFwf41RpLgtE7";

    var cache = {};
    var container = $("#example1");
    var errorDiv = container.find("div.text-error");

    /** Handle successful response */
    function handleResp(data)
    {
        // Check for error
        if (data.error_msg)
            errorDiv.text(data.error_msg);
        else if ("city" in data)
        {
            // Set city and state
            container.find("input[name='city']").val(data.city);
            container.find("input[name='state']").val(data.state);
        }
    }

    // Set up event handlers
    container.find("input[name='zipcode']").on("keyup change", function() {
        // Get zip code
        var zipcode = $(this).val().substring(0, 5);
        if (zipcode.length == 5 &amp;&amp; /^[0-9]+$/.test(zipcode))
        {
            // Clear error
            errorDiv.empty();

            // Check cache
            if (zipcode in cache)
            {
                handleResp(cache[zipcode]);
            }
            else
            {
                // Build url
                var url = "http://www.zipcodeapi.com/rest/"+clientKey+"/info.json/" + zipcode + "/radians";

                // Make AJAX request
                $.ajax({
                    "url": url,
                    "dataType": "json"
                }).done(function(data) {
                    handleResp(data);

                    // Store in cache
                    cache[zipcode] = data;
                }).fail(function(data) {
                    if (data.responseText &amp;&amp; (json = $.parseJSON(data.responseText)))
                    {
                        // Store in cache
                        cache[zipcode] = json;

                        // Check for error
                        if (json.error_msg)
                            errorDiv.text(json.error_msg);
                    }
                    else
                        errorDiv.text('Request failed.');
                });
            }
        }
    }).trigger("change");
});


//]]>

You can see an example here - http://www.zipcodeapi.com/Examples#example1

Since the question is how to autofill a valid city and state based on the user entering a zipcode first, the above Javascript code does that. Click on the example above to see how it works.