Yes, we all understand the goal for the national post. USPS charges the same for a first-class letter no matter what and has some flat-rate options based on size. But even they discriminate based on location on most packages. And we're talking about private sellers who are incented to minimize costs, using private shipping companies.

It might be different in the US but in most places I know, the national carrier is cheapest and the private ones like FedEx or TNT are used for special requirements, like faster shipping or special dimensions. There are a few cheap private shipping companies but they are also flat rate.