Card testing works like this. A thief buys a list of stolen card numbers. To find out which ones still work, they run them through a small store’s checkout with cheap orders, often hundreds in a row. Your store is the testing ground. You pay the fees.
The cards that work then get used for bigger fake orders, sometimes in your store. If you ship them, the real card owner disputes the charge, and you lose the product and the money.
Shopify flags some risky orders, but it leaves the decision and the cost to you. Skyfense adds the missing piece: whether the person behind the order acted like a real shopper or like a bot.