Instagram users in Indonesia have approximately increased by 50 percent in 3 years, which shows the growing popularity of Instagram in today's digital society. The role of influencers and engagement can be utilized to market the product. Those issues are expected to be the basis for conducting case studies related to marketing through social media. The objectives of this study are 1) analyzing the characteristics of followers of influencer XYZ's Instagram account who make product purchases, 2) identifying costs, convenience, influencers, engagement, and purchasing decisions of the marketed products, 3) analyzing the effect of cost, convenience, influencers, and engagement towards product purchasing decisions and formulating managerial implications to increase Instagram engagement and ultimately be able to increase marketed product purchasing decisions. The sample was determined by purposive sampling with a total sample are 320 respondents. Data obtained through Google Form were analyzed using descriptive analysis and Structural Equation Modeling (SEM). The result of the model shows that a) influencers have a significant influence on engagement, b) influencers have a significant influence on purchasing decisions, c) convenience has a significant  influence on purchasing decisions, and d) engagement has a significant influence on purchasing decisions. Influencers should focus on creating attractive and/or needed content by the target market consistently; maintaining convenience, especially in terms of availability, security, and speed in the purchasing process; and periodically measuring the engagement rate (ER) as an evaluation material when applying digital marketing, so the influencers are able to keep their attention on social media.