You need to remove your API Key and List ID from the code!

Thanks! New update available fixing that issue… ??

In __construct() you defined:

$this->mailchimp_api_key = $this->get_option( 'mailchimp_api_key' );
$this->mailchimp_list_id = $this->get_option( 'mailchimp_list_id' );

So, just use:

$mc = new MailChimp( $this->mailchimp_api_key );

$result = $mc->call('lists/subscribe', array(
    'id'         => $this->mailchimp_list_id,
    'email'      => array( 'email' => $object->billing_email ),
    'merge_vars' => array(
                        'FNAME' => $object->billing_first_name,
                        'LNAME' => $object->billing_last_name
                    ),
    'double_optin'      => false,
    'update_existing'   => false,
    'replace_interests' => false,
    'send_welcome'      => false,
));

No need to load the options twice ??

Thanks! Changed! ??

??