Online service brands launching an extension service (e.g., e-commerce platform) must determine whether their customer-brand relationships and brand equity will induce existing customers to have a favorable attitude toward the extension service and a strong intention to accept that service. This study focuses on mobile shopping platforms to determine the factors that drive users of a mobile messaging service brand (LINE) to intend to accept the brand’s newly launched extension service, i.e., a new mobile shopping platform (LINE Mart). Drawing upon the attachmentaversion (AA) model of customer-brand relationships, we propose a research model to investigate the impact of consumer-brand relationships (brand attachment and brand commitment) on brand equity, attitude toward the brand extension, and intention to accept the brand extension (acceptance intention). Data collected from 347 LINE users provide strong support for the research model. The results indicate positive associations among components of consumer-brand relationships and brand equity. We find that brand commitment is positively associated with attitude toward the brand extension, which in turn has a strong effect on acceptance intention. We also find that perceived fit has a positive effect on attitude toward the brand extension, and brand attachment has a positive effect on acceptance intention. We discuss the implications for theory and practice, and provide suggestions for future research.