I've been trying for the past week or so to get OPEN75UP to work (they sent me the suggestion as a new customer I guess), but it doesn't seem to work on anything at all. The error saying that the code only works with some publishers or 'item isn't eligible', or whatever, isn't very useful when you have no idea what that error applies to and have to go through the checkout procedure for EVERY BOOK to find out. Very annoying!
Not sure i'll bother with the Kobo store - clumsy search and expensive anyway, as well as this issue.