USPS Wordpress plugin question

Hey guys, got a few questions.

I am designing a site for a client & they want their shopping cart to calculate shipping costs and factor them into the checkout cost. Im using WP E Commerce right now and it is not calculating the shipping cost. Any suggestions?

Does anyone know of any Wordpress shopping carts that support USPS shipping?

WordPress Shopping Cart Plugin
http://tribulant.com/plugins/view/10/wordpress-shopping-cart-plugin