the shippable column wouldn't really work since you may different product types than cards that are shippable.
you said before that you had a idcategory_product that could be used to set the product category, use that column in the discount.
set the discounts up similar to what I suggested before using number of Items with a specific column value for the trigger.
If there are 5 cds, in the cart discount the price of one, if there are 6 cards in the cart, discount one card.