Facebook is pretty limited in the layout and content customization they will allow on their site lately. Even the workarounds we had previously recommended to add our cart buttons to a Facebook Page apparently no longer work, and we haven't yet found a new workaround that works -- if anyone's figured out a way, we're open to suggestions!
At present, the best we can recommend is just using your Facebook Page as-is, and find some way in there to include a link to a page on your Web site where your cart buttons would appear, or you can set up an E-junkie Shop page and link to that. E.g., you can put link URLs in your Facebook Page's About or Info sections, or in the caption for a photo.