I use a pair the tried and trued Wald folding baskets with liner bags. Carries a weeks worth of food. The metal baskets keep the grocery bags (flimsy plastic these days not paper) secured and the liner bags make it easy to bring everything in. All for around $60 for the pair (basket and liner bags). There are a couple nylon soft sided "grocery" panniers out there but they're much more expensive at one pannier for the price of the Wald pair (with liners). Only downside is that they're noisy when empty.