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.
I am writing to extend my sincere gratitude for ________(感谢的原因). If it had not been for your assistance in _________(对方给你的具体帮助), I fear that I would have been_________(没有对方帮助时的后果).
Every one agrees that it was you who__________(给出细节).
Again, I would like to express my warm thanks to you! Please accept my gratitude.