I think people are very, very wary about pre-checked checkboxes just under payment details, because it reeks of cramming. That is, they set off alarms not just

I think people are very, very wary about pre-checked checkboxes just under payment details, because it reeks of cramming. That is, they set off alarms not just that they might be subscribed to some email list, but that a "free trial subscription" (that auto-renews to a paid subscription) is being added to the order.