Falsely declaring a customers order as a GIFT is illegal.
For example, items acquired by means of a purchase are not considered a "bone fide gift" by US Customs Border Protection. Therefore they will not consider an item being shipped from a business to an individual as a gift. This applies to all countries you may ship to.
Should you mark an order as a Gift, customs may review your parcel. They will then contact the customer to request a copy of the invoice. Full duties and taxes may then be payable on the true value of the goods.
See information on "Bone Fide Gifts" to the US and more on the AusPost website here. This also applies to other carriers used to ship internationally.
Likewise, in many countries/regions, SAMPLE cannot be used as a declaration to ship items unless it is in an unsaleable state (eg. marking or mutilating the item in a way that renders it unsaleable)
To ensure your parcels are complying, always declare the true Reason for Export - which is SALE OF GOODS.
Information on complying with shipping to the EU can also be found on the AusPost website here