Problems figuring out %'s

So today is discount wine day.  This means that 99.99999% of our wine is discounted today. The customers get 10% off of their wine today.  So a customer came in and order a bottle of wine.  She received her discount and her total came to $8.82.  The normal price is $8.99 +tax which comes to $9.80.  She complained that her discount was wrong.  She thought that 10% of $8.99 must be at least $10. It was explained that 10% of something can't be more than the actual total of the item. She wanted to know how I came to this conclusion and I told her, "math". She looked at me with her best Jon Gruden eyes and said I was a tricky white boy. She was going to go home and check on my conclusion to make sure I didn't rip her off and then she would call me. She said I had better not have ripped her off.  I didn't think she'd call but I was wrong.  She and her daughter came to the same conclusion...."You owe me some money"...apparently 10% of something is bigger than the original total.