Sackville SaddleSack- Small
If you don't want the overkill of the Sackville medium or large - meaning you're not trying to haul the kitchen sink home with you - then the Sackville small is for you.
The Sackville small will fit your 13" Macbook and Macbook Air, some light groceries, a stack of books from the library, flower cuttings for your mother, and/or snacks for the road. This is a great commuter bag. In fact, we can't think of a single reason not to own this bag. And it's damn handsome to boot. Coincidentally, it loads and unloads with more ease than lacing up your boots.
You don't need to use a rear rack with this bag - it's too small for that nonsense. The bag is so well constructed, its entirely self-supporting. No rubbing on the tire here.