Unfortunately we have no way to do that for a specific product, though you could just create a separate account for that product and enable the Only authorize... payment preference there. Note that even with that preference enabled, we'd still process the order immediately when the buyer finishes checkout, so you'd only be delaying final settlement of their payment. You can add a Confirmed secondary email to your PayPal account to use as the PayPal Email for the new account, so PayPal payments from both accounts would go to the same place.
