A monthly trip to Costco has become routine this year for Kim Eun-young. The 42-year-old mother of two living in Seoul’s Gwangjin district used to pick up what she needed at a nearby supermarket, but rising prices prompted her to change her shopping habits. She said buying groceries and household essentials in bulk seemed more economical, at least for her family of four. “The bill is bigger each time, but buying in bulk works out cheaper for the same amount. I mainly buy food we eat regularly...