I keep a cargo net in my panniers, full time. If I buy a bit too much for the panniers, I can still get everything home. Plus, some sorts of staples don't fit easily in anything except an Extracycle's panniers. Charcoal, bulk rice, bulk flour... A cargo net makes it much easier to get them home.
If I know I'm doing a giant stock up trip, I'll bring a spare canvas bag along so I don't end up with plastic.