Are you an inverter packaging specialist? If they won't ship you the replacement first so you can use the box to send back the defective unit, place it in any box that it will fit in, slap the return label on it and send it back. Under the Magnuson Moss Warranty Act of 1975 it is the manufacturers responsibility to pay for ALL return shipping costs of a defective product and one could argue that would also include the cost of packaging.
Be weary of their request if it’s the vendor I think it is. They have a propencity to claim everything works fine once they have it back in their possession and then you're on the hook for return shipping. Demand that they send you the replacement inverter first to avoid this situation.