About once a week, maybe more. Visiting multiple stores (where it's already annoying to lug your prior purchases in with you) or just buying something like toilet paper will quickly achieve this qualification.
Even if cars were only used by each person a couple of times a week that's still a massive improvement over commuting everyday. (I also think you're exaggerating, toilet paper can easily fit in a backpack.)
Nearly all the grocery chains in the UK let you order online and have the goods delivered.