I keep an REI Round Town or whatever collapsable grocery bag pannier on my rack, i rarely need it, but it provides enough room to stop by the grocery store on the way home. most importantly it folds cleanly against the side of my rack completely out of the way when I am not using it...

I also bought a reusable REI grocery bag for $1 that I keep folded up inside it so when I go into the store I just grab that and go in. I put all all my groceries in it, since I know if they fit in the bag they will fit in my pannier....