Sorry missing your point...so what am I missing?
The fact is that most of the time, shipping via plain priority mail is cheaper, unless you are shipping heavy items....you think USPS is "giving away $$$?"
There are times flat rate is a wonderful bargain, if you are shipping heavy items. But, for the most part they came out with flat rate as a "no brainer" option for people who do not want to "hassle with it."
I ship several packages a week, mainly via USPS and it is rare that flat rate would be cheaper for my particular package. If I want peace of mind, I go UPS because they are much more reliable, although usually a couple of bucks more.
Unless I go overnight, FedEx is my last choice because I find their ground service for my particular packs to be less reliable, although they get there eventually.
And, again, I will point out that this is mostly pointless, because the OP can easily figure out the best option for his/her particular package by going to the USPS, UPS, FedEx Web sites and entering in the exact data and finding out pretty much to the penny how much it will cost.