Post office working hours vary. There may be some post offices open 24 hours a day in very big cities. However, most post offices are open from 9:00 A.M. to 5:00 P.M. There may be different hours on different days. The post office is open on Saturday until noon, if it is open at all.
There are many different types of mail.
Standard Mail: Generally used by big companies that do marketing by mail.
Parcel: Self explanatory.
Certified Mail: If you would like to be able to track the delivery of the mail, you can send it by certified mail. You will be given a tracking number that you can track online or calling USPS. It is a bright green color form. If you are sending certified mail, the same thing is called Registered Mail and it is a pink color form.
Of course, you will have to go to the post office in person to pick up the mail, and you will have to show your photo identification to receive it.
Return Receipt: If you would like the recipient to sign that they received your mail, you can send it by certified mail with return receipt. It is a dull green color form.