I think it's in Zola's novel about a Parisian department store (?Au Bonheur des Dames??) that he mentions that the shopgirls were underpaid on the grounds that they had the opportunity to meet men of a higher social class and become their petite amies. Or it might be in a relatively recent historical study on women, shopping and the rise of the department store during the C19th, the title and author of which at the moment completely elude me.