I already use a backpack, but I can only carry so much in it. I only buy for myself, and I don't mind a couple trips per week. I just need more space. I don't tour either. Budget isn't much of an issue as I'm working full-time now and car-free. It's hard to say how much space I need. Sometimes I get a huge watermelon. Then, there's no room for anything else, lol. I don't get watermelons much anymore. I intend on getting a gallon of milk and a couple dozen eggs with a pack meat or so for each trip. If running low, I'll get some fruits, veggies, cheese or whatever. I eat a lot probably enough for two people. The Kroger grocery store is less than 2 miles away, and the the majority of the route residential houses. I buy stuff in glass, so I rather not have a basket with stuff falling out. Would too much weight on one side of the bike be too awkward, or should I get two panniers?