The way to sell note cards and other non-fine art items is not using categories, as Sunny suggested, which is a way of organizing your fine artwork in your DPW Gallery website. Instead, you should create an item store page, which you ca read about here:
I've looked at your DPW Gallery website and can see you already know this and already have a "Note Cards" store items page, however I am writing this to inform other members who may read this post.
To answer your question about whether you can create quantity based discounts in DPW for your store items, the answer is no. You could, however, create another item that is a collection of cards and offer that "item" at a discount.