Social commerce relies heavily on user engagement, yet the cognitive and behavioral drivers of content sharing remain unclear. This study integrates the Interest-Search-Mouth-Action-Share (ISMAS) consumer journey with the Heuristic-Systematic Model (HSM) to propose a dual-process framework. Analyzing 3,604 product notes from RedNote, we examine how heuristic cues (e.g., cover design, emojis) and systematic validation (e.g., comments, creator credibility) influence sharing behavior. Results show that heuristic simplicity (e.g., single-image covers, declarative titles) captures initial attention, while systematic trust signals (e.g., comments, bookmarks, grassroots creators) drive actual sharing. Interestingly, content from business accounts with large followings reduces sharing, reflecting user skepticism toward commercial messaging. The ISMAS-HSM integration advances understanding of social commerce and offers practical guidance: content creators should balance visual appeal with authenticity, while platforms should promote community-driven trust. This work bridges cognitive and behavioral theories, offering actionable strategies to enhance engagement and foster meaningful interaction in digital environments.