Magento – Why checkout_onepage_controller_success_action is dispatched after the layout is loaded

If this were a standard ORM model these events would likely exist. Because the Mage_Checkout_Model_Cart model extends Varien_Object instead of Mage_Core_Model_Abstract, these events are dispatched by hand, including the passing of the cart object into the event scope. This allows for similar event-based injection of processing logic related to the cart object before and after the quote save process.

I've had a similar problem (removing dynamically payment methods) and solved it by extending the _canUseMethod($method) function in Mage_Checkout_Block_Onepage_Payment_Methods. There you can return false for specific payment methods which should not be shown in the payment selection.

Example:

class Company_Module_Block_Methods extends Mage_Checkout_Block_Onepage_Payment_Methods {
    protected function _canUseMethod($method) {
        if ($method != null && $method == 'checkmo') {
            $carriers = Mage::getSingleton('checkout/session')->getQuote()->getShippingAddress()->getShippingMethod();
            if ($carriers == 'flatrate_flatrate') {
                return parent::_canUseMethod($method);
            } else {
                return false;
            }
        }

        return parent::_canUseMethod($method);
    }
}

The config XML would look like:

<config>
    <global>
        <blocks>
            <checkout>
                <rewrite>
                    <onepage_payment_methods>Company_Module_Block_Methods</onepage_payment_methods>
                </rewrite>
            </checkout>
        </blocks>
    </global>
</config>

That way I could remove payment methods right before they get shown in the one page checkout.

Hope that helps anyone.