As the lady behind the counter has a very heavy accent it is hard to hear exactly what she is saying, but its more likely that she said $20 because as you yourself pointed out she only handed her one bill.
Actually, when Sara is paying for stuff, it's the first trip to the store for her. On that first trip, she got the Book of Shadows, The Craft, and some candles. The price quoted was 50 dollars.
Wrong! Its not $30, and its not $50... She clearly says "Twenty dollars" Read it on the sub titles, or listen, thats a novel idea... Her accent does not make it hard to hear twenty... Blessed Be