The dust is an achievement to re-enter the shop if you have the fairy at home you will get a hint to talk to her for help.
buy 10 watermelons and give it to the fairy
To re-enter the shop, you first need to make sure you have first hidden the amulet and then spiked her drink, if you then get too large to enter you get told to speak to the fairy at home, the fairy will tell you to bring her 10 watermelons
