Chicago's Consumer Services Department recently finished its latest survey of city stores, looking for unintentional mark-ups at the cash register. Their report shows that, while most supermarkets and retailers are improving their error-catching rate, an alarming 78 percent of Chicago drugstores are overcharging its customers. The department's tips on preventing overcharging are common sense ones: bring sales ads to the store; mention any overcharges right away to the cashier; and check your receipts for discrepancies.