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Include a copy of your driver's license
Include a note stating who the gun is for and provide their phone number.
Pack the gun so that it can not move around inside the box. It's best to break guns down (assuming some sort of double gun) and wrap the barrel section and stock section with bubble wrap. Pad around those sections with wadded up newspaper or more bubble wrap (Styrofoam peanuts suck). Again, the gun should not be able to shift or move inside the package.
Insure the shipment for the full value.