Magento – Uploading Extension in Magento Marketplace

It is strange! I just copy your logic and test it, not worked after that I start to debug model load flow, edited and review logs. So finally it worked! But my code same as yours: Customweb_Saferpay module:

    <models>
        <saferpay>
            <class>Customweb_Saferpay_Model</class>
            <resourceModel>saferpay_mysql4</resourceModel>
        </saferpay>
    </models>

Etagen_SaferpayGuestOrderFix module:

   <models>
        <etagen_saferpayguestorderfix>
            <class>Etagen_SaferpayGuestOrderFix_Model</class>
        </etagen_saferpayguestorderfix>
        <saferpay>
            <rewrite>
                <method>Etagen_SaferpayGuestOrderFix_Model_Method</method>
            </rewrite>
        </saferpay>
    </models>

Method.php:

class Customweb_Saferpay_Model_Method
{
    public function getPaymentPageLink()
    {
        return get_class($this);
    }

}

Second one:

class Etagen_SaferpayGuestOrderFix_Model_Method extends  Customweb_Saferpay_Model_Method{

}

Testing code:

$model = Mage::getModel('saferpay/method');
echo $model->getPaymentPageLink();

But result sometimes correct, sometimes incorrect (incorrect after cleaning of cache). Finally I found such xml config and it gives correct result alltime:

    <models>
        <etagen_saferpayguestorderfix>
            <class>Etagen_SaferpayGuestOrderFix_Model</class>
        </etagen_saferpayguestorderfix>
        <saferpay>
            <deprecatedNode>etagen_saferpay</deprecatedNode>
        </saferpay>
        <etagen_saferpay>
            <rewrite>
                <method>Etagen_SaferpayGuestOrderFix_Model_Method</method>
            </rewrite>
        </etagen_saferpay>
    </models>

Who reads this question, please, test it and I can't find reason of such behavior of Magento. Location of models are located in same code pool and maybe this is reason of such understandable situation.

This is the part that makes problem here.

    <Mycompany_OrderExport>
        <active>false</active>
        <codePool>local</codePool>
        <depends>
             <Xtento_OrderExport />
        </depends>
    </Mycompany_OrderExport>

You have disabled your module by setting the active node as false. Turn it on. Means put true for active. Make sure module is enabled through admin side. You can do this by go to

 System  >  Configuration  >  Advanced

Clear all cache and then try again.