don't quite understand you.
but if the custom fields are set as mandatory, it means we want these fields completed. having members to navigate to the configuration page just to fill it up is counter-intuitive. members will forget or not know what they shld do next after purchasing.
instead after purchasing, it cld bring to a new page with all the items and their custom fields (highlighted whether mandatory or not), so members will know which fields MUST be completed, and which are optional.
besides, i don't think members purchase 100's of items at one go. I wld suspect most ppl purchase 1 item each time with a majority buying not more than 10 at a time.