Add a little "Lulu-only" icon to the format options which are NOT available for wider distribution.
Problems with the lack of an ISBN option when a Lulu-only print format has been picked by mistake continue to dog the forum questions. The little green tick is great when it appears but does need a learning curve to notice what it means. A "Lulu-only" icon against the other options (perhaps a little green "L" for Lulu in a circle?) would draw attention to what it means.