More questions about shipping more than one card to keep shipping costs low for the customer

[cards-in-the-closet]

Hi all,

I would like to charge .99 cents shipping for the first card, then .15 for each additional card, up to a MAX of $6.00 for my low-end cards, but I can't figure out how. I did find the Flat Rate Shipping Discount page and saved .15 cents for each additional item, but I'm not sure how to apply it, and apply it selectively to just card listings because I sell more than cards. Any advice?