Für einen aktiven Check muss ein Observer in /etc/config.xml registriert werden:

<frontend>
    <events>
        <payment_method_is_active>
            <observers>
                <vendor_modulename>
                    <type>singleton</type>
                    <class>Vendor_ModuleName_Model_Observer</class>
                    <method>activeCheck</method>
                </vendor_modulename>
            </observers>
        </payment_method_is_active>
    </events>
</frontend> 

Der ActiveCheck kann anschließend wie folgt in der /Model/Observer.php durchgeführt werden:

/**
 * Checks risk.
 * 
 * @param Varien_Event_Observer $observer
 * 
 * @return null
 */
public function activeCheck(Varien_Event_Observer $observer)
{
    $event      = $observer->getEvent();
    $methodCode = $event->getMethodInstance()->getCode();
    
    $buergelApi = ... 
    if ($buergelApi->checkRisk($methodCode, true)) {  // perform api request here 
        $observer->getEvent()->getResult()->isAvailable = false;
    } 
}

Um den Passive Check zu integrieren, kann die OrderAction des OnepageController wie folgt verändert werden:

/**
 * Create order action
 */
public function saveOrderAction()
{        
    $buergelApi = ... // load buergel api class
    $methodCode = $buergelApi->getQuotePaymentMethod();
    
    if ($buergelApi->checkRisk($methodCode, false)) {
        if (!$this->_validateFormKey()) {
            $this->_redirect('*/*');
            return;
        }

        if ($this->_expireAjax()) {
            return;
        }
        
        $session = Mage::getSingleton('checkout/session');
        $session->setData(
            'vendor_modulename_disabled_method',
            $methodCode
        );

        $result['error']          = true;
        $result['error_messages'] = __('The payment method is currently not available.');
        $result['goto_section']   = 'payment';
        $result['update_section'] = array(
            'name' => 'payment-method',
            'html' => $this->_getPaymentMethodsHtml(),
        );

        $this->getResponse()
            ->setBody(Mage::helper('core')->jsonEncode($result));
     } else {
         parent::saveOrderAction();
     }
}