Usung button_to to send product_id back to the controller along with additonal info entered through a form on the same page as the button

Question

I am extending the shopping cart model (from Agile Web Development with Rails) that lets a user 'add to cart' a product_id through the session. The id is only stored in the database (as a line_item) after a user clicks 'checkout'.

Id like to attach other unique data related to that line_item, like color and special_instructions, through a form on the same page as the 'add to cart' button. However, I've read that it isn't a good idea to pass lots of info like this through the session. Is this right?

If it is the proper approach, how do I store any additional data in the session before it is passed to the shopping cart? I can't seem to figure this out.