This is actually a bug/limitation in dnn when you redirect to another user control dnn applies the admin skin instead of the portal skin.
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/107/threadid/46342/scope/posts/Default.aspx
This is a technique we have been working on but it’s kind of a big change and will require a lot of testing before its release and we are hoping that dnn will fix the bug so we dont have to implement this workaround.
http://www.r2integrated.com/dnn/Forum/tabid/170/aff/1/aft/4732/afv/topic/Default.aspx
Long story short you will need to set your portal and admin skin to the same skin temporarily if you want the same skin on the cart page.
-Scott