In re-reading some of Campbell today, I realized that Sophie-as-hero crosses the threshold to adventure the moment she stuffs that mushroom-pleated hat in the wastebasket.
She’s already told the hats she’s sick of them, that “you certainly aren’t doing me a scrap of good.” She’s already thought the unthinkable thought: “Someone has to do this or there will be no hats at all to sell…. Does it matter if there are no hats to sell?”  And she’s just gone and broken the unbreakable law of retailers everywhere, namely that the customer is always right:
The rule was: Lose your temper, lose a customer. She had just proven that rule. It troubled her to realize how very enjoyable it had been. 
Without quite realizing it, Sophie has just done for the family business. Without taking a step, she has left the life she has always known and crossed into the Unknown.
And right on cue, the sea-monster/ogre/dragon/sphinx that always guards the threshold entrance appears, in the form of the Witch of the Waste.
One had better not challenge the watcher of the established bounds. And yet — it is only by advancing beyond those bounds, provoking the distructive other aspect of the same power, that the individual passes, either alive or in death, into a new zone of experience….
The adventure is always and everywhere a passage beyond the veil of the known into the unknown; the powers that watch at the boundary are dangerous; to deal with them is risky…. [Hero, 82]
Though she is not at first aware whom she is dealing with, Sophie “sees” well enough magically to known that the elegant, “carefully beautiful” woman who just came breezing is not as young as she seems, and that the sad, terrified little person who follows her in like a whimpering puppy is “clearly younger than the lady…. Perhaps the lady was his mother.” [24-25]
“I hear you sell the most heavenly hats,” said the lady. “Show me.”
Sophie did not trust herself to answer in her present mood. She went and got out hats…. She followed Fanny’s advice and got out the wrongest first.
The lady began rejecting the hats instantly. “Dimples,” she said to the pink bonnet, and “Youth” to the caterpillar green one…. “Mysterous allure. How very obvious. What else have you?”
Sophie got out [a] modish black-and-white, which was the only hat even remotely likely to interest this lady.
The lady looked at it with contempt. “This one doesn’t do anything for anybody. [Of course it doesn’t; Sophie had made it quite clear to that hat that it wasn’t doing her a scrap of good!] You’re wasting my time, Miss Hatter.” 
Sophie is still boiling from her earlier encounter with the customer who returned the mushroom number, and she’s boiling now. Is there no end to these people with their expectations and their demands? “Only because you came in and asked for hats,” she snaps back. 
Sophie believes the lady is dissing her hats, when in reality she is dissing Sophie’s spells. The Witch thinks Sophie is some other Miss Hatter who has set herself up in competition to the Witch, a Miss Hatter who is “meddling with things that belong to me.” 
And, in what is probably the most famous scene in the novel, the Witch delivers the curse that turns Sophie into an old woman, then breezes out.
“By the way, you won’t be able to tell anyone you’re under a spell,” she said. The shop door tolled like a funeral bell as she left.
Sophie put her hands to her face…[and] felt soft, leathery wrinkles…. She pulled her gray skirt against her legs and looked down at skinny, decrepit ankles and feet which had made her shoes all knobbly. They were the legs of someone about ninety and they seemed to be real. 
They are real. And this Witch is a really nasty piece of work: a human woman who has enjoyed the privilege of an unnaturally prolonged life-span, willfully shortening another woman’s life by at least half a century. But why kill her victim outright, when instead the Witch can torment her by robbing her of her youth, her beauty, any love she might have known, any children she might have had, all the potential opportunities and satisfactions of her life, not to mention her health?
For a young woman who was vain and invested in her own looks and talents, this would be a funeral bell tolling indeed. Sophie, however, is a very different sort of young woman, and the Witch should have done her homework a little better.