Questions 46～48 refer to the following letter.
Theater Digest
125 Lake Ave
Chicago, IL 6o6i6
Dear Subscriber,
    Your subscription to Theater Digest will end in two months. Please don't let that happen. Take time to renew your subscription today. By doing so, you will continue to receive every month the way latest in theater reviews, information on actors and directors, and up-to-date reports on new dramas and musical for the next year.
    I've attached an invoice for your renewal order. You will receive 12 issues for the special low price of $35. Please send your payment in the reply envelop provided. Make any corrections to your name or address fight on the back of the invoice. Then visit our web site at www.theaterdigest.com to read about contests for readers. You could win tickets to great show!
      What is the purpose of this letter? ______.