The ‘hidden cost’ tactic adds in high charges to users, and it is revealed only during purchase completion. For example, online travel agencies' websites display the headline price as the total price. However, when customers purchase the products/ser- vices, additional costs (taxes, booking fees, resort fees, etc.) are gradually added (Nota, 2019). The ‘hidden cost’ dark pattern tactic exploits consumers' cognitive bias called ‘sunk cost fallacy’. Sunk costs are those that cannot be recovered once incurred because consumers have invested resources in the form of money, emotion, effort, or time (Emich & Pyone, 2018). Since the consumers have invested their resources in searching for hotel rooms, they go ahead with the hotel booking suggested by the online travel agency and thus fall into the ‘sunk cost fallacy’ trap.