If you want to give users the exact shipping cost and not have to figure out how to do it with Flat Rate Shipping, you can get rates directly from USPS. That way, you can't estimate the costs wrong and lose money on shipping. It takes a bit of extra work to set it up, but the results make it very easy for users to select the exact shipping method they want and there's no confusion for the admin.

You could also get rates from UPS or FedEx and the process is very similar. The reason I choose to use USPS is because it's slightly easier to set up. With USPS, WooThemes has been able to include credentials in the plugin, which means you have one less account to set up.