“ March 3, 1883, § 7, (22 St. p. 523,) declaring that, in determining the amount of customs duties for which goods are liable, the usual and necessary boxes or coverings of any kind in which they are imported shall not be taken into consideration: provided, that if any coverings be designed for use otherwise than in the bona fide transportation of goods to the United States the same shall be subject to an ad valorem duty of 100 per cent. ”