I would argue that a postal service is not structured the same way as an on demand service like uber.
A postal courier who arrives at your door, picks up an important document, and takes it straight to the recipient will cost about the same.
When you write a letter or send a parcel you first take it to a designated pick up point. It is then picked up at an allocated time along with all the other letters and parcels, and at best it is going to arrive the next day having been through a huge sorting routing system at the post depot.