The postal service of St. Lucia, an island in the West Indies, obtains a significant portion of its revenues from sales of special souvenir sheets to stamp collectors. The souvenir sheets usually contain several high-value St. Lucia stamps depicting a common theme, such as the anniversary of Princess Diana's funeral. The souvenir sheets are designed and printed for the postal service by Imperial Printing, a stamp agency service company in the United Kingdom. The souvenir sheets cost the postal service $0.60 each. (The currency in St. Lucia is the East Caribbean dollar.) St. Lucia has been selling these souvenir sheets for $5.00 each and ordinarily sells 50,000 units. To test the market, the postal service recently priced a new souvenir sheet at $6.00 and sales dropped to 40,000 units