Are you paying for the cards, or are they a "no-cost-to-you" item? My assumption is that you do purchase them, so you have an expense and increase in physical inventory. When you sell the service, you are "including" the card as a "no-cost-to-customer" part - correct? I would stock it in as an inventory part. At time you sell the service include the card as a line item on the invoice at time of sale as a zero price to customer.