Sewn bags are not waterproof, you have to put stuff in plastic bags inside, at least.

there are welded seam bags from several makers . one that would be useful ..

this is a single bag with a watertight zipper opening
http://www.ortlieb.de/_prod.php?lang...eshopper&pf=1a
http://www.ortlieb.de/_prod.php?lang...nt=bikeshopper Accessory back pack pad clips on,
http://www.ortlieb.de/_prod.php?lang...nt=bikeshopper

Its the same format as their excellent touring rear panniers, which are sold in pairs ,
with a different top closure system.

sewn backpack with a hidden covered pannier hook set , at Nashbar.
may be more affordable .
I have a set of Ortlieb panniers .. amongst the best.
(argument will alway be raised).

Of course you can fit a big front basket , like from Wald, on the bike and put the messenger bag in it.