With as much time as you spend walking, carrying your day's necessities, personally I would avoid a single strap courier bag, I think they are way top uncomfortable for prolonged off bike/large load portages. I'd go with a two strap model of some kind. Have an uber yard sale, then drop $240 on a Bailey Works "Citizen" @ 1,764 cu. in., or for your needs, the "2-Strap Work Pack," which tops out at 3,7780 cu. in. for a mere $300!