A conundrum to solve please

I have the Freddy’s food app and it gave me a code for a birthday concrete - thick thick milkshake. I gladly accepted and got it and my son and I enjoyed it. The app SHOULD have then declared the code null and void and taken it off. A week or two after my birthday, it was hot and we went back there to buy a milkshake. My son went through the app and said daddy - the code is active - we can get another one! I was shocked that it was active and we went through drive thru and I showed the lady the code, she zapped it with her laser and it accepted it! She also wished me happy birthday, which I should have said that it had already passed and that I had used the code before….but I didn’t. I thanked her and we enjoyed the shake.

Now the conundrum - Was this theft? Should I have told them that I had already used it and they should fix the app? Should I go back and pay them the cash equivalent of a free birthday shake of $5.99? Some may say let it go, it was a mistake on their part, and a stroke of good luck on mine? I just don’t know what to do. I’m thinking of going back and paying them the $5.99. What would you do?