No shipment is allowed to have a value of zero (these are international shipping rules set by the universal postal union)… even if it is a gift / freebie! that is why there are specific deminimis rules when the end_use of a package is indicated as gifts, personal or business that differ. let me know if you have further questions.
is the product price zero? why dont you use a discount code so that it doesnt affect the MSRP instead?