Easy answer - Ortleibs. Lift them off the bike, carry them into the store, toss them in the cart. Shop. Ask the clerk to put my groceries back in my cart. Load the bags myself so heavy and sturdy stuff goes into the front bags that go on a LowRider rack, lighter and bigger stuff in the rear bags. Get home and carry the bags into the kitchen. Any spills inside a pannier are as easy to clean as any pot. (Oh, and once the top is rolled shut, my food will stay fresh far longer if the weather turns hot and I take my time getting home.)
Ben