I get your point on the USPS. However, neither UPS nor FedEx (or any other carrier) deliver letter mail. Non-USPS carriers can deliver parcels and time sensitive documents, but not regular letter mail. This is due to the Private Express statute. At this time no carrier is trying to overturn the Private Express statute due to the enormous equipment and personnel overhead it would take to do so.

Similarly, the reason both FedEx and UPS have agreements to pay the USPS deliver items to the "last mile" is likely due to the lack of profitability to deliver low-cost items by FedEx and UPS drivers. USPS carriers deliver mail to most every address every day in the US. FedEx and UPS tend to deliver larger parcels and express items to significantly fewer addresses daily.

One could argue that the USPS is subsidizing Amazon, eBay and other online retailers by allowing other carriers to use USPS personnel for the last mile delivery. If UPS and FedEx had to deliver all of their smaller parcels themselves, they'd have to increase the number of drivers, trucks, sorters and sorting equipment. Because smaller parcels tend to be less profitable for FedEx and UPS, they'd likely have to increase the costs of these smaller items. This would then make it more expensive for online retailers to mail merchandise to their customers, and these costs would be passed on to customers.

If I were the CEO of a brick-and-mortar company, I'd be constantly complaining to my congressman about the Federal Government subsidizing online retailers.