Magento – How to recalculate shipping costs in Magento one page checkout

There isn't a clean interface to do that, but the following hack works well in any Magento version.
First declare an observer for the sales_quote_collect_totals_before event.

<config>
    <global>
        <events>
            <sales_quote_collect_totals_before>
                <observers>
                    <your_module>
                        <type>singleton</type>
                        <class>your_module/observer</class>
                        <method>salesQuoteCollectTotalsBefore</method>
                    </your_module>
                </observers>
            </sales_quote_collect_totals_before>
        </events>
    </global>
</config>

Then, in the observer, adjust the handling type and fee configuration for each carrier in the corresponding store model.

public function salesQuoteCollectTotalsBefore(Varien_Event_Observer $observer)
{
    /** @var Mage_Sales_Model_Quote $quote */
    $quote = $observer->getQuote();

    // Check your customer attribute of choice here instead of the customer id
    if ($quote->getCustomer()->getId()) {

        $store = Mage::app()->getStore($quote->getStoreId());
        $carriers = Mage::getStoreConfig('carriers', $store);
        foreach ($carriers as $carrierCode => $carrierConfig) {

            // F for fixed, P for percentage
            $store->setConfig("carriers/{$carrierCode}/handling_type", 'F');

            // 0 for no fee, otherwise fixed of percentage value
            $store->setConfig("carriers/{$carrierCode}/handling_fee", '1');
        }
    }
}

Note that the configuration changes are not saved, they are only valid during the request. But because they are applied before the totals are collected, they will be taken into account whenever the shipping rates are calculated.

By default that isn't possible, you must, at least implement:

• A custom Shipping Method
• A custom Shipping Form for your method, to allow customer select different shipping method for each item.
• Create sales_quote_item and sales_order_item attributes to save customer method selection

Your custom shipping method must handle all that logic, you need to split all items/weights and calculate the individual/total shipping cost/price and eventually some combination too ie:

- item 1 by usps = $10.
- item 2 by usps = $10.
- item 1 and 2 by usps = $15

Your shipping method must be able to:

• Obtain the shipping cost/price from the others shipping methods (for each single or combination possible item selection)
• Save customer shipping selection to sales_quote_item
• Recalculate all that data in all further steps in checkout
• Save quote_item data to order_item
• Re-implement any other -parent- shipping method feature, like Api calls for pickup, packages generation or tracking number getters

Finally if you want to give your customers a good communication feedback, you should modify all "post place order" email and templates in order to display all that information.

IMO is a lot of work to do, and I can't figure out a real-life case in which such effort would by justified.

EDIT

I'm not totally sure, but a partial use of some dropship extension may help... I guess