The letter of the law may exempt you according to the specific case and local laws, but I have been taught by my rabbis to pay. And when you point out the oversight to the agent and offer to pay, you fulfill a mitzvah and collect a greater payment than you made.
This mitzvah is called “Kiddush Hashem” — literally, “sanctifying the name of God” by doing a religious or moral act that causes others to revere God. The reaction of a person who sees this special act is “The Jewish People display the righteous and pleasant ways of their God’s teachings, and they should be truly happy!”
Two brief personal stories. My wife and I placed an order for an electrical appliance in Israel that needed to be imported from the States. When it arrived to our home it came with a receipt that stated it had already been paid for (although we did not). And the cost involved was more than our monthly budget! We asked our local Rabbi who told us we needed to pay for it. When we went to the office to pay, the employee insisted it had been paid for, and only after re-checking the paperwork saw that indeed no payment had been made. After consulting with the owner of the company he told us that they would not accept any payment from us. Our “honesty” was more than enough payment. Our Rabbi said we should nevertheless give a fifth of the amount to charity.
On another occasion we took our young children on a miniature-train ride during a holiday. We purchased tickets and gave them to the children who proceeded to get on and ride the train. They also got off the train at the end, along with their tickets, saying that the person didn’t ask for their tickets and begged us to let them ride again with the same tickets. We said we needed to speak with the employee-in-charge. He looked at us and said, “This is the first time anyone has been so honest about the ride! I see that you are Orthodox Jews, and that must be the reason. It is my honor to let your children ride another time with those tickets!”
