update-billing-postcode-via-select-woocommerce.php

<?php

add_action('wp_footer', function(){
    
    if(!function_exists('is_checkout') || !is_checkout()) {
        return;
    }
    
    ?>
    
    <script>
        (function($){
            
            var $doc = $(document);
            var $body = $('body');
            
            $doc.ready(function(){
                
                function updateBillingPostCode(e){
                    var $this = $(this);
                    var value = $this.val();
                    // console.log(value);
                    
                    var $target = $('#billing_postcode');
                    if($target.length > 0) {
                        $target.val(value);
                        $body.trigger('update_checkout');
                    }
                }
                
                $doc.on('change', '#billing_address_type', updateBillingPostCode);
                
            });
            
        })(jQuery);
    </script>
    
    <?php
});