Its not a monthly commission rate. The ECN rebates come in at the end of the month. For example: If your rate is .005 and you add liquidity (using ECN rebate of .002), the firm will charge you as if you took liquidity (making it .003 charge for taking liquidity plus your base rate .005 so thats .008 ) . Then at the end of the month, you get a .005 rebate (the .002 you earned plus the .003 your firm charged you for assuming you took liquidity).