The use of psychological findings in EU internal market regulation has gained interest, particularly in the area of unfair commercial practices. This study investigates consumer perceptions of bonus packs containing an oversized indication of the Bextra^volume in the package, such as in the Mars case. The Mars case serves as a standard reference in EU unfair commercial practices law which is used as a benchmark to determine the Baverage consumer.^Our study demonstrates how an experiment can be set up to provide empirically based insights on whether a practice is Bdeceptive.^Results of our experiment show that consumers overestimate the extra volume when confronted with an oversized indication compared to control conditions, which is first empirical evidence that this practice is potentially deceptive.