If you don't mind not having the site design on the ordering page, you could theoretically put it together outside of your WP, upload it, and then link to that URL from your WP. Or you could look for a cart or order form plugin for WP that supports Authorize.net that you can just install. Other than that, I'm afraid you need at least basic knowledge of how to add to WP templates to implement something like this - you probably need to find a WP programmer and pay him for a few hours of consulting work.
