You need to apply the Server validation server behavior to the forms action page.
In this case, the action page is the confirm page, so the Validation needs to take place there. Set the Redirect on Validation failure to go back to the checkout page.
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.