Abstract: In some examples, a fulfillment service may receive a purchase order for delivery of one or more items. In response, the fulfillment service may determine an initial risk score representing a first probability that the purchase order is a potentially fraudulent transaction. In addition, the fulfillment service may request fulfillment of the purchase order based on the initial risk score being below a predetermined threshold. During fulfillment of the purchase order, the fulfillment service may monitor one or more attributes associated with the purchase order. Further, the fulfillment service may determine a revised risk score based on monitoring the one or more attributes. In some cases, the fulfillment service may cancel fulfillment of the purchase order based at least in part on the revised risk score.