"You could specify the to and return adress, you could pay with a credit card, and you could add whatever options you want."
What would the point of obfuscating your address with a 16 digit ID number be? You'd be adding another layer of complexity to an already simple system of names and addresses on a designed letter carrier's route. If you want obscurity you can always register a PO box.
Your solution also relies on other businesses easily and quickly adopting your idea, which in reality is never quick and easy unfortunately.
Truth be told, the mail system isn't perfect, but USPS has done a lot in the last 3-4 years to innovate their own business. I don't have to wait in line anymore because I can print out postage and tape it to my package and leave that at the post office.
Unfortunately a lot of post office customers don't seem to realize the online system exists.