I've read several pieces in which doctors say that lugging overloaded backpacks is a cause of back pain in young people. That aside, first step is to stack up your typical load for weighing and measuring. The pack should accommodate the largest item/stack of items without straining the zipper. The heavier the load, the more stiffness and structure you need. Some failrly small packs have built in plastic frames and waistbelts.
If it's hot, you want a pack that will ride slightly away from your back, so you don't end up sweaty and sticky. If you carry items that provide padding (extra clothes) they should go next to your back, with the stiff things (books, tools, laptop) outboard. Lunches and things that shouldn't get squashed ought to have a spot on top or a separate pocket. Or use a bento box, etc.
We lived in Auckland, New Zealand, without a car and I did most of our shopping with a 1.5 mile walk and a return trudge up a long slope. I used a super-light, unstructured backpack from Go-Lite that I'd gotten when I wrote gear reviews. It was usually stuffed tight enough to ride away from my back. I'd pack cans and bottles in the bottom, then stiff packaging (boxes, cartons) in the middle, with produce and bread up top.
If you try retail shops, take your stuff along and give each pack a real-world test.
Good luck!