The checkout line at this Walmart was going to be very slow: ahead of us were four grown-ish children and their mom, and their cart was packed to overflowing.

“How about you bring the car around for my dad,” I suggested. “You guys wait, and I’ll text when I’m through.” My husband nodded, and the two of them headed out.

Between me and the family with the packed cart was an older couple; behind me was a younger couple. All of us had just a few things—I had a laundry basket, a bathroom scale, and a s...