If you've got a relatively decent scale, you can figure out shipping online. I always get a shipping estimate before I list an item, so I can cut down on my trips to the post office (no scale here).
On a sort of related note, I found out this week that you can't send something First Class if it weighs more than 13 ounces! I don't know why I didn't know that - guess most of my stuff was just skating under the limit, since I've been sending books first class for a while now.