law: Indian Post Office Act, 1898 
  13. Customs duty paid by the Post Office to be recoverable
  
  as postage
  When a postal article,
  
  on which any duty of customs is payable, has been received by post from any
  
  place beyond the limits of 5[India], and the duty has been paid by
  
  the postal authorities at any customs.-port or elsewhere, the amount of the
  
  duty shall be recoverable as if it were postage due under this Act.