I use the Topeak trunk bag, the DXP which is the largest one they make. It has fold out panniers. I use that and my Chrome Bravo backpack, which goes from 20L to 40L. Between that and the trunk bag all my groceries are a done deal. And yes, the trunk bag has a removable shoulder strap so you can take it with in the store. You need Topeak rack to use the bag. It slides in and out on the rack using a rail and locks and unlocks the bag. Check it out on Amazon.
http://www.amazon.com/Topeak-Trunk-B...peak+trunk+bag
The Topeak rack,
http://www.amazon.com/Topeak-6310703...opeak+mtx+rack
