Quote from jimrockford:
Commission costs for shorting are the same as for other trades, but sometimes, on some dates, for some stocks, you must pay an extra interest charge for the privilege of borrowing the shares on that particular date. This is done in cases where IB must pay extra in order to borrow the shares for your use. A broker who lends out a customer's shares, or its own shares, so that they can be shorted by the borrower of shares, is entitled to charge the borrower interest on the borrowed shares.