Any given product consists of whatever you wish it to be, so you could simply sell the ebook with the understanding that product includes a card to be mailed to the buyer. Make sure you enable Shipping/Buyer's Address in the product's settings to trigger collection of a mailing address during checkout.
If you want to show the card as a separate item in the buyer's cart, you might consider creating a multiAdd to Cart button:
http://www.e-junkie.com/bb/topic/2121
However, in this case bear in mind we have no way to prevent buyers removing the ebook item from their cart after they multiAdd the ebook and card, allowing them to place an order for the remaining bonus-card item without the ebook. Some sellers have devised custom javascript to handle this scenario -- e.g., if you go here, add an item to the cart, then add the backup CD option shown in the cart, removing the original item from your cart also removes the backup CD item:
http://www.muvenum.com/buy-now/