Most local branch post offices have their own zip codes. Some large office buildings that receive a great deal of mail also have their own codes. Don't forget to use the proper zip code. If you don't know it, the post office will have a directory where you can look it up. Classes of Mail.
American mail is divided into four classes.
You'll probably only use first and fourth class, since second and third class mail are for business and advertising use. First class is the fastest and most expensive. It is for the average letter and some matter sent in larger envelopes. If the item weighs more than 12 ounces, you'll be sending Priority Mail. It also can be slow.