Writing a plugin, need to display data on post.php action edit

I’m writing a plugin to work with a custom shopping cart. I’ve successfully hooked the plugin into the save_post action and can dump data to the screen after a post is saved. However right now its just throwing a session error. What I need to do is echo my string somewhere onto the wp-admin/post.php?action=edit&post=4115&message=1 page. I have not been able to find where to do this though. Is there a method of hooking my string variable into the “view” portion of the wordpress post page?