Is it wrong to remain silent when a cashier makes a mistake in your favor?

At a popular department store, I wanted to buy two items for $2.94 each and condoms for $14.00. The cashier was about my grandmother's age. She scanned the $2.94 items three times and said the total was $8.82. I knew the price wasn't right, , but I didn't want to say to the elderly woman, "Excuse me, but you didn't scan my condoms." I got a good deal, but I think that was somewhat immoral on my part. Is that right? What should I have done?