The postal acquisition is for vehicles that will be in use for 30+ years. In that time there won't be a Transit from Ford. Nor will many parts remain available. They cannot customize to maximize the efficient use as a postal vehicle because the Transit isn't designed to Drive 100 feet. Stop. Drive 100 feet. Stop. Drive 100 feet. Stop, 500 times a day, 6 days a week and be useful for handling in-truck package storage with minimum wear and tear on the postal worker who is handling it all. Also, UK Spec Transits won't comply with US DOT requirements, so even more customizing.
They could buy "this year's" vehicles and have 30 years worth of differing support tools, differing maintenance schedules, not to mention that each purchase is accompanied by a competition among the providers, not only for the vehicles, but also the customizations required for each one.
Note that Amazon has custom trucks and so does UPS. The one for Amazon is $80,000.
Of all the government agencies getting equipment the USPS has been the best and most efficient. They have to be. The volume of material they handle would fill entire warehouses in just a few weeks if they started to fall behind. They need systems that are tough and reliable and have been at this job longer than there has been a standing army in the US.