as you know I do drop shipping, bit hard for me to drop cards in there with the orders.
Good point, over looked that lol
You can make it cheaper then and do an electronic one, too easy. In regards to tight margins, offer a purchase two at special price just for you type deal, then you cover your margin on the first and sell the second at a reduced margin or give it away as you are still making your money on one.
It's a good way to introduce customers to new products, and is very effective if you can offer them on similar items to what they just purchased.
Just remember to allow for the difference in postage from your DS
You should also consider doing a monthly newsletter if your customer base is building. That's a great place to list bonus package deals, and of course with zero eBay fees.