参考答案：cheaper than letters

解析：

[听力原文]

Probably every town in the United States has a post office. Some are very small, and you may also find them in the corner of a shop. Others are large buildings. They are open five days a week and Saturday morning. From Monday through Friday they are usually open from 8:30 a.m. to 4:30 p. m..

If you know how much the postage is for your letter, you can buy stamps at any window. In some post offices you can buy stamps from machines. Stamps are sold at many different prices, from one cent to many dollars. If you are not sure how much postage is for your letter, you can ask the clerk. He or she will give you the stamps you need.

At a post office you can also buy postcards. A postcard is cheaper than a letter.