Ortlieb Vario Backpack/Pannier QL3
If you need to carry your pannier for any length of time, it's more comfortable to carry it as a backpack than as a shoulder bag - especially if it's heavy. So Ortlieb have designed the Vario, which is a roll-top backpack that quickly converts into a pannier. The carrying system (with two shoulder straps, a sternum strap and a waist strap) tucks away into a front pocket when you're using the bag as a pannier.
This bag uses Ortlieb's QL3 system, which leaves most of the hardware on the rack. This makes the bag easier to use as a backpack, and it also means you can attach it with one hand.
The capacity is 20L, with space for A4 documents and a 15" laptop.