Surprised no one mentioned Timbuk2 paniers. I have a set that cover all your needs. The look small when empty, and can hold vast amounts. Each one can hold a 12 pack - if that helps (also a requisite for me). They are quickly removed from rack and turn into a shoulder bag instantly. really cool. They're expensive ($129) but often go on sale at places like REI. Timbuk 2 seems to be moving away from bike stuff. They used to be featured on their website and now I can barely find them (and I think they changed their name). Anyway, sounds like it's too late for the OP but if anyone's reading this I would strongly consider.