Zack is guilty of theft by deception. He intentionally created a situation that he hoped would cause the cashier to charge less than the established amount. His plan worked. He knew when he paid that he was paying less than the established value. Although the cashier could have been more careful, her error does not excuse Zack from the criminal consequences of his trickery.
The difference between the Basic Situation and this Variation Two lies in the fact that, here, Zack intentionally set up the cashier's error whereas in the Basic Situation it (presumably) occurred more or less by accident. The crucial thing however is that both here and in the Basic Situation, Zack was aware of the error as the transaction was occurring.
In this Variation Two, Zack is more morally culpable than in the Basic Situation, since here Zack intentionally maneuvered events to cause the error. His greater moral culpability might merit greater punishment in the unlikely event the full facts were to come to light.