The current-gen mail vehicle is called the LLV - literally the Grumman Long Life Vehicle - and the mileage these things have taken on speaks well to the name.
USPS tends to order these things with the expectation that they'll last forever (400k+ miles) with relatively little upkeep. Most cars out today, even if they get to e.g. 200k miles, will probably have taken on some pretty major repairs.
Because when you're buying a "major car OEMs are happy to sell this number" number of vehicles you can afford to get exactly what you want and the resultant few % reduction in operating cost over the following decade or two will more than make up for the increased cost up front.
Also odds are this thing will be a mish-mash of GM or Ford parts bin with just the body and body specific stuff being different (like the LLV, or that handicapped van AM general used to make). So they may very well be buying a "customized Ford Transit" where it just so happens the body is also custom.
