I usually take a knapsack with me when I get groceries on my bike in case I don't have enough basket space and to carry fragile items. I'd like to get one with an internal frame because a frame will transfer the weight to the belt more efficiently so there is less strain on the shoulders. (When I got home from the store yesterday I found I had strained some muscles in my neck from riding with a heavy load in my knapsack).
Can anyone recommend an internal frame pack with a large capacity but which is not too large to wear while sitting on a bike? The top of my rear basket is about level with the top of the seat on my bike so a regular camping backpack would be too long - my bike is designed for the rider to sit upright. I will probably use the same pack if the weather was too snowy or icy and I had to walk to the store.