Planning ahead is important, since the automobile encourages those unplanned (and often unnecessary) trips that are less convenient on a bike or bus.
I do a lot of light shopping on my way home from other destinations. I pick up things I might need in a week or two, especially staples. For example, yesterday I went to the supermarket to buy supplies for a spaghetti dinner. I realized I hadn't quite reached my carrying capacity, so I bought some cans of tomatoes and extra pasta to make efficient use of the space.
I have recently gotten in the habit of joining my son and his family in the minivan on their weekly shopping trip. This is a good chance to pick up bulky and heavy items. We often do another activity like a movie or lunch out, so it is fun for everybody.
I grant that on occasion carfree living is a little less convenient. But one compensation is that I will fully enjoy that extra trip to the supermarket on my bike, while it would be drudgery if I had to drive.