That's something already fixed that you're asking about. Exchanges charge a "fixed" fee for a transaction that they execute on their end and you are bound to pay those fees if you are withdrawing your coins from an exchange to exchange or even a wallet. You can only decide fee while you are sending "your" coins that you have in your wallet, and not an exchange. The "fixed" fee charged by them is what they take per transaction, not the fee they actually send while sending you your "withdrawn" amount. One more thing, each exchange has different "fixed" fee levels, like Polo charges 0.0005 BTC per withdrawal, Bittrex takes 0.001 BTC and so on.