Inaction inertia is the phenomenon in which people are less likely to accept an opportunity after having previously missed a relatively superior one. Based on previous studies of inaction inertia, psychological distance, and quantity promotions, the current research explores how missing different quantity promotion discounts (multiple discount vs. single large discount vs. single small discount) influences inaction inertia and relevant mediators. The results of three experiments demonstrate a missed quantity promotion with a single small discount (e.g., "buy 3, get 25% off"), compared with a missed single large discount ("buy 5, get 40% off") or a multiple discount (e.g., "buy 3, get 25% off, or buy 5, get 40% off"), inducing higher inaction inertia through enhanced perceived closeness, upward counterfactual thinking, and perceived regret.Compared with missing a multiple-discount promotion, missing a single largediscount promotion enhances inaction inertia. However, the effect is only mediated through the number of upward counterfactual thoughts and perceived regret. These findings indicate perceived closeness, and assignments to specific mental accounts are important internal mechanisms activating inaction inertia.