Concession Abuse as a Service (CAaaS) is a growing scam service in underground forums that defrauds online retailers through the systematic abuse of their return policies (via social engineering) and the exploitation of loopholes in company protocols. Timely detection of such scams is difficult as they are fueled by an extensive suite of criminal services, such as credential theft, document forgery, and fake shipments. Ultimately, the scam enables malicious actors to steal arbitrary goods from merchants with minimal investment. In this paper, we perform in-depth manual and automated analysis of public and private messages from four large underground forums to identify the malicious actors involved in CAaaS, carefully study the operation of the scam, and define attributes to fingerprint the scam and inform mitigation strategies. Additionally, we surveyed users to evaluate their attitudes toward these mitigations and understand the factors that merchants should consider before implementing these strategies. We find that the scam is easy to scale-and can bypass traditional anti-fraud efforts-and thus poses a notable threat to online retailers.