When you go to your nearest magic shop to buy ingredients, remember that proper etiquette is very important and try not to argue with the shopkeeper. He knows you are a powerful witch, but he also knows better than anyone what ingredients you need to cook a perfect potion, so keep a good relation with him.

BONUS: Seducing him is not an option to pay less for the ingredients, they usually wear magic glasses to avoid such deceptions.
Buying ingredients