This system option setting defaults to "on" (checked).

 

If disabled (unchecked), it will prevent any invoice from being printed if it has one or more items with a zero price and a non-zero Quantity Shipped (Weight Shipped for "LB." items).

 

The entrée system has no way of knowing when a price has been set to zero accidentally and when it has been set to zero on purpose, there are no exceptions to this rule.