Is this still considered stealing or just good luck?

So today i went to a hardware store to buy something which i did and i think the lady at the register forgot to scan something because the price was way lower then i expected. I knew she made a mistake right away but i did not say anything. I later checked my receipt and i was right she totally forgot to scan the item. It was only a $20 item but did i get lucky or could this still be considered stealing?

Is this still considered stealing or just good luck?
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