Final answer:
The best action is to remove the expired sale sign, apologize, and honor the listed sale price to maintain good customer service, then ensure all store signage is updated.
Step-by-step explanation:
When facing a situation where a customer finds an old, expired sale sign in front of a product and is charged a different price than they expected, the most appropriate course of action is to remove the sale sign, apologize to the customer, and honor the sale price listed on the sign. This response maintains good customer service by acknowledging the oversight and ensuring the customer is satisfied. Afterwards, it's crucial to check the rest of the store to make sure that all signage is current to prevent similar situations.