that looks like it should work as long a the hidden element is in a cart display and you are clicking the update cart button, that should do the trick.
can you send a copy of the page you are using this on and a link where I can see the problem.
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.