I have a seatpost rack and bag I would be willing to sell to you at a very reasonable price. It clamps right to the seatpost and you can take it on and off very quickly. But, like Oak said, you can't use it with a carbon seat post. You could always swap your carbon seat post out for aluminum when you are commuting if that's an issue. I bought this used on the forum for a bike trip I took last year and it's just sitting in my garage gathering dust.

I tried the backpack when I started commuting and quickly gave it up and started looking for better options. Besides being hard on your back, it's hot when you are riding in the summer.

The rack is similar to this and has a trunk bag like this that slides on and off.