If you have a commercial account with a supported carrier, you can offer your customers the convenience of choosing that carrier during checkout. The rates are automatically downloaded, so you do not need to look up the information.
Before you can offer your customers a selection of shipping carriers, you must first complete the shipping settings to establish the point of origin for your store. Then, complete the configuration for each carrier service that you want to offer. The configuration options vary for each carrier. However, all require that you first open a shipping account with the carrier, and enter your account number or user ID, and the gateway URL to their system into the configuration of your store. See Magento Connect for additional shipping services for your Magento Open Source installation.