Final answer:
The sales associate should apologize and open a register promptly to address the customer's complaint.
Step-by-step explanation:
In this situation, the best way for the sales associate to address the customer's complaint is to apologize and open a register promptly. It is important for the associate to acknowledge the customer's frustration and take immediate action to resolve the issue. By opening a register and attending to the customer's needs, the associate can provide better customer service and improve the customer's experience.