This extension automatically applies shipping based on 3 parameters:

COUNTRY: When a customer first go to the shopping cart, shipping method is calculated using the store’s default country (System->Configuration->General->Countries Options->Default Country).

If the customer is logged in, the country of the customer’s shipping address is used. If a customer selected a country and clicked ‘get quote’, the selected is used.

SORT ORDER: Sort order of shipping methods can be set up at System->Configuration->Shipping Methods->Shipping Method Name->Sort Order Shipping method has the smallest sort order value will get applied.

For examle, Set Free shipping to have a sort order of 1, Flat Rate to have a sort order of 2. When freeshipping is available, it will be applied. When it is not available, flat rate is applied.

POSTALCODE When a customer first go to the shopping cart, shipping method is calculated using the store’s default postalcode (System->Configuration->Sales->Shipping Settings->ZIP/Postal Code).

If the customer is logged in, the postalcode of the customer’s shipping address is used. If a customer selected a postalcode and clicked ‘get quote’, the selected is used.

Shipping rates will get recalculated when the customer:

• add items to cart
• update items in cart
• remove items from cart

https://github.com/Geert82/Magento_AutomaticShippingCalculation/wiki