Get the tx fee for a tx
    Return the fee on success
    Return None on error

def get_tx_fee(tx_hex, config_path=None, bitcoind_opts=None, bitcoind_client=None):
    """
    Get the tx fee for a tx
    Return the fee on success
    Return None on error
    """
    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)