It is possible that the shipping value is somehow reset. That code appears as though it should work unless the cart was cleared or a session variable that was used is reset.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.