As mobile payment becomes increasingly widespread, promotional activities play a crucial role in driving its adoption. In a two-sided market involving both merchants and customers, mobile payment providers offering promotional discounts to users also significantly impact merchants. This study analyzes five months of sales data from 40 chain convenience stores to examine the effect of user-targeted promotions by mobile payment providers on store sales performance. During this period, Tencent launched a WeChat Pay promotion offering random discounts to users who made payments on Tuesdays. Based on this, the study applies the Difference-in-Differences (DID) method to analyze the impact of WeChat Pay’s random discount promotion on merchants’ sales performance. The results indicate that random discount promotions have a significant positive impact on merchants’ sales performance on the day of the promotion. Further analysis rules out the possibility that this sales growth is due to consumers delaying or advancing their purchases. Additionally, the study examines the dynamic effects of the promotion, investigating how the impact of the promotion evolves over time to address the question of the optimal promotional duration for merchants. Finally, the study finds that the proportion of payments made using WeChat Pay moderates the effect of the promotion on sales performance.