I love the Arkel Commuter bag. It's big enough for a change of clothes, my lunch and some shopping, and I put the entry card for the bike room in the little zip pocket on top, my phone in the zip pocket in the side, and the shoulder strap in one of the side pockets. I do find I have to carry it backward though, with the hook facing out, when I'm using it as a shoulder bag.
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Zero gallons to the mile