What a very very honest person your are !!!!! It was nice that Joann's let you keep the extra fabric without charging you more, or having you send it back to them.
I have done that myself many times in the past with mostly employers giving me more than I was suppose to get in my paycheck. I was always told to keep the extra money as a reward for being so honest. Then I had no guilt feelings. The extra money was appreciated. I sometimes worked for some very cheap doctors and they let me keep the money. WOW !!!! It does pay to be honest.