You can trade for FREE with coinbase but you have to use their exchange called GDAX. And you also must be a "maker" in the market which means when you place your buy or sell order you will need to set it at a price between the bid and ask. They dont charge you anything because you are creating liquidity in the market. But if you buy or sell at the current bid/ask price, you are taking from the market and there will be a small fee.