Introduction Alcohol consumption among young people is strongly related to alcohol availability. The minimum legal drinking (purchasing) age (MLDA) is a legal measure that regulates alcohol availability to minors in Slovenia. This study examines (1) retailers’ compliance with the MLDA law in Slovenia and (2) the effectiveness of two interventions directed at cashiers in off-premise stores. Methods The study uses a non-randomized quasi-experimental design to evaluate the effectiveness of (1) a communication intervention directed at off-premise store managers, and (2) an intervention by the Slovene Market Inspectorate. The first intervention focused on informing cashiers about MLDA’s importance and their role as gatekeepers of young people’s health, while the second involved law enforcement. Using the mystery shopping protocol, we conducted two waves of purchase attempts with decoy underage shoppers pre- and post-intervention in 97 off-premise stores. We collected data on the shopping process at the point of sale and conducted 40 semistructured interviews with cashiers to evaluate the barriers and incentives regarding MLDA compliance. Results Retailers’ initial noncompliance rate with MLDA in off-premise stores was high, but improved significantly after the law enforcement intervention. We identified a significant correlation between the cashiers’ ID requests and the refusal of alcohol sales, but cashiers’ ID requests remained low. Qualitative findings reveal that cashiers experience several issues when handling MLDA in practice. Conclusion Noncompliance with MLDA persisted even after the law enforcement intervention, revealing the need for policy makers to introduce new strategies for MLDA enforcement, such as revocable alcohol licenses for off-premise stores.