Good backpack for carrying groceries So, I need to be able to carry 3-4 days worth of groceries for one person, about a mile from the store to home. I'm not too keen on the idea of panniers or anything else that mounts to the frame. I don't want any extra junk burdening me during the 99% of rides that aren't grocery runs. So, I've been wearing a backpack but its a little small and not well suited to carrying groceries. Does anybody have a suggestion that will work in my use case?

Get an ortleib messenger backpack classic. It will hold a ton of stuff and it's waterproof in case something bursts inside (it will stay inside).

If you want to go real fancy get a mission workshops vandal. It is a great multi-purpose bag and it expands to fit a lot of stuff. Watch the video demo: Grocery Run - Vandal Roll-Top Backpack

You may also want to consider a quick release handlebar basket, depending on whether you have drop bars or not.