Disallow a PO Box in WooCommerce

disallow-po-box.php

<?php

add_action('woocommerce_after_checkout_validation', 'deny_pobox_postcode');

function deny_pobox_postcode($posted) {
  global $woocommerce;

  $postcode = (isset($posted['shipping_address_1'])) ? $posted['shipping_address_1'] : $posted['billing_address_1'];

  $postcode = strtolower(str_replace(' ', '', $postcode));

  if (strstr($postcode, 'pobox')) {
    wc_add_notice( "Sorry, we don't ship to PO BOX addresses.", "error" );
  }
}