Rack-top bag or panniers? Yes!
This expanding design lets you go around town or out of town. It keeps a low profile until you need the drop-down panniers and expanding top. An unexpanded capacity of 900 cubic inches is already pushing twice the volume of many rack-top bags. But with the top up and panniers down you have 1300 cubic inches.
A detachable shoulder strap makes it easy to carry when it's not on your bike.
Attach a safety light to the web loop to supplement the 360 degrees of reflective accents.