You can create an Administrator account (as opposed to the Super Administrator) and have your client use that for login. They would be able to update products, view orders, and update content, but would not be able to update the design.
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.