Can i take advantage of people's mistakes

I often had a temptation which was getting a miscalculated bill. I could discover easily that the cashier missed one or two items which i purchased. It meant i paying less. I had to go back to the cashier, corrected the number and paid again.
Today, I went to Taipei with my sweet daughter. The event happened again.
Since Frances is a twelve-year-old young lady, she needed to buy an adult ticket. I did not know about it until we left on the journy. I was not ready for it because she is just a KID.
All right, the temptation came. On the way back, the train station clerk sold Frances a child ticket. I paid without second thought. Because it was really cheap. After boarding, I thought about the amount of the money. I knew she made a mistake. What should i do?
Frances is still a kid. It should be fine to get a child ticket, isn't it?. Is it true? By the law of the train station, Frances should buy an adult ticket. But she is JUST a kid. I struggled. There was a voice coming out. " By the law of the train station...". I knew it was my God to remind me.
Without hesitating, I told Frances that i had to pay extra money for her ticket because of the mistake. I needed to declared it immediately or i would be in doubt again.
When we got off the train, i found a station agent and told her about the situation. She looked at Frances and told me, " It is all right for this time but please buy an adult ticket next time.".
Oh! What a relieve!
It was a good lesson for Frances as well. I told her if i did not tell the truth, i would have owed the train station few dollars and would be haunted by it forever. It was very unwise just for few dollars to ruin my life.
My God always loves me so so much that He reminds me to do the right things. Praise Him with all my thankful heart!