  We study the statistical laws of the expenditure of a person in convenience
stores by analysing around 100 million receipts. The density function of
expenditure exhibits a fat tail that follows a power law. We observe the Pareto
principle where both the top 25% and 2% of the customers account for 80% and
25% of the store's sales, respectively. Using the Lorenz curve, the Gini
coefficient is estimated to be 0.70; this implies a strong economic inequality.
