if you have an invoice or legal paper you can send deliver it yourself, you can send it by private held company like TNT, UPS, whatever but only when you send it via Polish Post (national operator) it gets so called the power of postal stamp. Legally if you choose the right delivery type it is valid as delivery in court. Such postage is still deeply embodied in legal system and I think it has some merit.
Exactly. In the U.S., many federal and state laws assume that the United States Postal Service will be there. Furthermore, the day the item is postmarked is, for most legal documents, considered to be the day the court receives it. Thus, if you have to have something filed by a certain date, you can delay (like most people do) and wait until the absolute last minute, run to the post office, and get a manual postmark put on the envelope, the only way you're going to get a guaranteed legible one in the U.S. Furthermore, courts rely upon the postal service to deliver legal mail. Federal courts allow you to file legal paperwork (as well as get copies of it) online and some state courts are slowly moving in this direction, but there will always have to be a way to get a piece of paper to someone who is a luddite or, is in jail or prison and does not have access to the Internet, and vice versa, since Americans have a constitutional right to access to the courts.