You might need one bike bag for commuting, one for shopping, one for family rides and so on. Or, you can get the multi-versed carry (almost anything) Dayliner Box Bag. It attaches to the top of your front or rear rack, handlebars or fits most bike share baskets. The removable shoulder strap allows you to conveniently carry 20 Litres in between rides. Made for total utility on any outing with a large roll top closure for overpacking.