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<sales_quote_collect_totals_before>
    <observers>
        <!-- Set default shipping method and rate when necessary -->
        <autoshipping>
            <class>My_Module_Model_Observer</class>
            <method>setQuoteShippingMethod</method>
        </autoshipping>
    </observers>
</sales_quote_collect_totals_before>

<?php

    /**
     * Has shipping been applied to quote?
     *
     * @var bool
     */
    protected $_hasShipping = false;

    /**
     * Set shipping method and rate if they do not exist yet
     */
    public function setQuoteShippingMethod()
    {
        if(!$this->_hasShipping) {

            $this->_hasShipping = true; // This is to avoid loops on totals collecting

            $quote = Mage::helper('checkout/cart')->getQuote();
            if (!$quote->getId()) return;

            $shippingMethod = $quote->getShippingAddress()->getShippingMethod();
            if ($shippingMethod) return;

            $shippingAddress = $quote->getShippingAddress();
            $country = 'FR'; // Some country code
            $postcode = '75000'; // Some postcode
            $regionId = '0'; // Some region id
            $method = 'tablerate_bestway'; // Used shipping method

            $shippingAddress
                ->setCountryId($country)
                ->setRegionId($regionId)
                ->setPostcode($postcode)
                ->setShippingMethod($method)
                ->setCollectShippingRates(true)
            ;

            $shippingAddress->save();
            $quote->save();
        }
    }