When do the post offices open in the US?

Every town in the United States has a post office. Some are very small, and you may find them in the corner of a shop. Others are larger buildings. (17) They are open five days a week and on Saturday mornings. From Monday through Friday they are usually open from 8: 30 to 16: 30.
    If you know how much the postage is for your letter, you can buy stamps at any window. (18) In some post offices you can buy stamps from machines. Stamps are sold at different prices, from one cent to many dollars. If you are not sure how much postage is for you letter, you may ask the staff in the post office for help. (19) If you are sending your letter far away. you should use airmail envelopes. Remember that postage will be more expensive for a letter to be sent outside the country.
    At a post office you can also buy postcards. (20) Usually the price of postage for a postcard is about half that of a letter. The postcards that you buy at a post office do not have pictures and cannot be sent to the places outside the country.