Another option if you have a rack on your bike is to use a single pannier on the rear rack. That's what I use when I commute to work, and I have also used it on long day rides when I wanted to carry "stuff" with me. I use a small pannier - which is actually classified as a front pannier - on the back. And it really doesn't make the bike feel off balance. I actually prefer the feeling of the weight down closer to the ground - I hate the feeling of a handlebar bag. (Yes, I know, that might be considered a bit odd.) Just another option to think about...

--- Denise