The purchased shoes arrive directly to him, of course. He couldn't risk having them delivered straight to her and having the shopkeeper know he'd bought something for Hermione Granger. Knowing what the box contains, he doesn't open it. He squirrels it up to his dorm to inspect the shoes in person, where he puts a complicated sizing charm on them. Within a close enough margin, they'll fit to her feet perfectly. And he thinks he got close enough.