remove all other shipping method if there's a method having shipping amount zero

Question

Could it be possible to remove all the shipping methods except the one having shipping amount 0?

There may be a promotion rule which will return the amount 0 for shipping, so it must not be removed.

Answer 1

Display only shipping methods with price 0 (zero) if any

In my module I'd overridden the Mage_Checkout_Block_Onepage_Shipping_Method_Available block as:

config.xml

<global>
    <blocks>
        <checkout>
            <rewrite>
                <onepage_shipping_method_available>Excellence_Subscription_Block_Checkout_Onepage_Shipping_Method_Available</onepage_shipping_method_available>
            </rewrite>
        </checkout>
    </blocks>
</global>

Excellence_Subscription_Block_Checkout_Onepage_Shipping_Method_Available

<?php

class Excellence_Subscription_Block_Checkout_Onepage_Shipping_Method_Available extends Mage_Checkout_Block_Onepage_Shipping_Method_Available {

    public function getShippingRates() {
        $groups = parent::getShippingRates();
        $free = array();
        foreach ($groups as $code => $_rates) {
            foreach ($_rates as $_rate) {
                if (!$_rate->getPrice() > 0) {
                    $free[$code] = $_rates;
                }
            }
        }
        if (!empty($free)) {
            return $this->_rates = $free;
        }
        return $groups;
    }

}