Dear Mr Cooper,

　　I work at the Friendship Bookstore on Dongfang Road where you often come to buy books.The other day when you bought a dictionary at our bookstore I made a mistake abut the price.You looked at two different editions of the same dictionary,one costing 30 yuan and the other 21 yuan.You took the 21-yuan but I accepted 30 yuan from you.I didn't find out my mistake until you had left.Will you please come to the bookstore  to get your change or to return your dictionary and get a 30-yuan edition instead? I apologize for whatever trouble this is causing you.

　　Yours truly

　　Li Hua