Poor dietary habits pose the greatest global health risk. Online grocery stores offer new possibilities to implement behavioral interventions that support healthier choices. For example, persuasive message prompts can encourage consumers to reflect on their food choices. Personalizing reflection prompts with self-references and delivering them with the correct timing can increase their personal relevance, which the Elaboration-Likelihood-Model suggests may boost their persuasive effectiveness. In two within-subject experiments, we found personal relevance and perceived threat to freedom to significantly predict persuasive effectiveness. Personalization through self-reference boosted relevance, but not persuasive effectiveness, while timing made no difference. Open-ended participant feedback identified perceived manipulative intent and a poor fit to individuals’ situations as key pain points. To improve the effectiveness of persuasive message prompts encouraging healthier food choices, we propose refinements to reduce reactance and enhance goal specificity.