Controller Script not Able to Get AJAX Data - php

Question

I am trying to pass serialized form data to a PHP controller with jQuery. On successful processing, the user will be redirected to a results page. I am not seeing any redirect taking place which tells me something is wrong with my controller. I am seeing a sucessful AJAX callback.
The parameters are 'zipcode' and 'token'. The controller will validate the token, die if invalid, or perform the necessary logic once if valid.
I cannot find anything that stands out as incorrect about my code and could use an extra set of eyes.
HTML
<form action="" class="zipsearchform">
<div class="form-group">
<h2>Search by Zip Code</h2>
<label for="zip-search-field">Enter a Zip Code</label>
<input type="text" class="form-control" id="zip-search-field" name="zipcode">
<br>
<input type="hidden" name="token" value="<?php if (isset($token)) echo $token; ?>">
<button type="submit" class="btn btn-default zipsubmitbtn">Find Notaries</button>
</div>
</form>
JS
$('.zipsubmitbtn').on('click', function(){
event.preventDefault();
var params = $('form.zipsearchform').serialize();
$.ajax({
type: 'post',
url: 'parseByZip.php',
data: {paramString: params},
async: true,
success: function(){
alert(params);
}
});
});
PHP Controller
<?php
include_once 'resources/functions.php';
$zipcode = $_POST['zipcode'];
$token = $_POST['token'];
if (validate_token($token) && isset($zipcode)) {
$pattern = '/^\d{5}([\-]?\d{4})?$/';
if (preg_match($pattern, $zipcode)) {
$radius = 25;
include_once 'resources/apizip.php';
header("Location: notaries.php?lat=$latitude&lon=$longitude&distance=$radius");
} else {
echo "Zip code field cannot be empty and zip code must have the proper format of 00000 or 00000-0000";
}
} else {
echo 'Unable to handle your request right now.';
}