Just add this after line 71:
That way it will update when they press the checkout button as well and you can solve the problem without adding the keypress event.
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.