A small comment on the admin page for this module. As for usability its not that good that you can have the panes for rearanging pages and the panes next to each other as this might not flow in some layouts - it wouldn´t be a problem but the functionality breaks if you take away the floats in the css.

I would suggest either to put the panes section after the pages section. Another - and usibility wise better solution would be to put a small button next to the pages that says "edit panes".

At the moment if I want to drag a page before the other the panes edit pops open and I can´t drag stuff around anymore. Otherwise I like the jquery integration - seems very well done.

BTW I have narrowed the error I mentioned yesterday. If you leave any field open in the shipping adress (I have only shipping adress no billing adress defined to make it less confusing to the customers) that is required it goes - rightly so - back to the page and asks you to fill in the missing information - thats alright. But then if you fill in the missing information and want to continue you get the following error:

"An illegal choice has been detected. Please contact the site administrator."

clicking on "next" again gets you to the next page and everything works as expected. Couldn´t figure out yet why it throws you the error.