The Rainbow Grocery's coupons, printed in local phone books, offer 20 percent off on Wednesdays and Thursdays, the San Francisco Chronicle reported Saturday. On those days, shoppers have to wait for a parking space and then face long lines at checkouts.
Karla Camer, who has become more cost-conscious since losing her job, said her father gives her his coupons for many months, allowing her two discount shopping trips.
"I never miss a month. Twenty percent is a lot of money when you are spending a couple hundred dollars," she said.
Larry Gilmore, a network administrator at the store, said the coupons' popularity was sudden.
"It has caused growing pains from all angles, from simply getting the stock on the shelves to ordering and being able to predict how much staff we need to schedule and how many people we have to hire," he said.
Rainbow's employees make decisions jointly, and they are trying to decide whether to add more days when the coupons can be used or to cut back on their value.