This addon calculates shipping from the location closest to the customer. There is no limit to the number of shipping locations that may be used. The closest shipper is calculated by the Great Circle distance based on Zip/Postcode. It requires a database listing the Zip/Postcodes and their latitude and longitude for the country/countries that you will be shipping to/from. A current US Zip code database is included.
The forum support thread is at http://forums.oscommerce.com/topic/350859-addon-closest-shipper/