code: def get_tx_fee(tx_hex, config_path=None, bitcoind_opts=None, bitcoind_client=None):
      
      tx_fee_per_byte = get_tx_fee_per_byte(config_path=config_path, bitcoind_opts=bitcoind_opts, bitcoind_client=bitcoind_client)
      if tx_fee_per_byte is None:
          return None
  
      return calculate_tx_fee(tx_hex, tx_fee_per_byte)
docs: Get the tx fee for a tx
      Return the fee on success
      Return None on error