Hello, fellow ATLien!
I use one of
these*. It's big enough for my gigantic 15" work laptop plus a change of clothes so it should be fine for your stuff, and it's super cheap. The downsides are that it's not totally water-
proof (mostly because of the design, since it doesn't roll shut like an Ortlieb) and that if you hit a big enough bump it's possible for it to become partially dislodged. Nevertheless, I'm more than satisfied with it considering the price.
I only have one of them; I attached a Wald folding basket to the other side of my rack for carrying things like groceries.
My commute is about 30 minutes driving (with traffic), or 30 minutes by bike on the Beltline. Not a hard decision about which is better!
(* Note that Performance Bicycle has a local brick & mortar store, at I-85 and North Druid Hills Rd.)