E-commerce conversations are increasingly multimodal, yet end-to-end response scores cannot localize which dialogue competence failed, and no public benchmark evaluates finegrained multimodal dialogue understanding for Korean commercial agents. We present KoMMERCE, a Korean multimodal fashion-domain benchmark that coordinates nine fine-grained dialogue understanding tasks on the same conversations: sentiment, intent, and strategy classification;image and text knowledge retrieval;dialogue state tracking; span detection; coreference resolution; and the first formulation of cross-modal dialogue relation extraction. The corpus contains 15,293 dyadic role-play dialogues under two-stage quality control. Baselines on six frontier multimodal LLMs reveal task-conditional patterns that should inform deployment choices: no model wins all nine tasks, and context length affects accuracy differently from one task to another.