The post office is mean. They have your mail, and thus they know they can be as mean as they want, cause what are you going to do?
Case in point...my mother sent me a package. Now, she knows that I have Fridays off. But she did not know that this Monday I had my day off instead. So she sent a huge package on Friday.
Not being home I got one of those irritating slips from the post office so I called to see if the mailman could deliver it again tomorrow. The guy on the phone was so rude. He hung up without saying good-bye. You know what it is? They don't have to be nice. It's not like you have a choice and can have someone ELSE deliver your mail. It's them or no one. They know it, you know it, and so they can be the biggest jerks.