A bottle of invisible drink.
Invisible drink is a potion made by Vendetta to turn anyone who drinks it invisible. She said that it took hours to make. Charlotte drank the entire bottle and it is unknown if that was all Vendetta made or if there was more at her house. At the end of the episode, it revealed Carl, Charlotte's imaginary friend, had a bottle of the liquid, so he must know how to make it, too, or Vendetta made it in the past and Carl drank it.