Override USPS carrier and implement free shipping based minimum grand total amount instead of subtotal

Override using below code (into config.xml)
<usa>
                <rewrite>
                    <shipping_carrier_usps>Packagename_Modulename_Model_Usa_Shipping_Carrier_Usps</shipping_carrier_usps>
                </rewrite>
            </usa>


Then create model file
/public_html/app/code/local/Packagename/Modulename/Model/Usa/Shipping/Carrier/Usps.php

and override only below method

public function getMethodPrice($cost, $method = '')
    {
        return $method == $this->getConfigData($this->_freeMethod)
            && $this->getConfigFlag('free_shipping_enable')
            && $this->getConfigData('free_shipping_subtotal') <= $this->_rawRequest->getPackageValueWithDiscount()
            ? '0.00'
            : $this->getFinalPriceWithHandlingFee($cost);
    }

//getPackageValueWithDiscount this is changed function

It's Done ~~