Yikes. You have to wonder how many other customers had the same issue, and possibly didn't even realize it. I hope you can get your money back without too much fuss.
I think it is the mindset of a younger generation, what with out mobile banking and the ability to access our account balance anytime, to not save receipts or balance our checkbooks. Why take the time to do the math, or save a receipt and deduct it from our balance when we can usually access our account and see the current balance within minutes of a transaction.
Many of my friends have this mindset. They will live receipts on the table at a restaurant, including the "customer copy", or leave receipts hanging out of the pumps at gas stations as they drive away. I am guilty of this myself, and only have begun holding on to things the past few years. My debit card was declined while trying to pay for gas awhile back. I tried it again, successfully, and pumped my gas. Later, when looking through my online transaction summary, I saw I'd been charged twice for that gas. I contact my bank and submitted a claim, worried the entire time that I would not be refunded because I could not provide a receipt for either transaction. Fortunately, my bank sorted it out and I got my money back, but it made me a lot more conscious about saving receipts from both approved and declined transactions.
I can't say I'm very good about balancing a checkbook, but I do save all my receipts now for practically forever now. This way if there is something odd on my statement, I can dig through 50 crumpled up receipts in my bag/floor of my car/desk drawer and eventually find what I need to sort things out.