Quite a few national postal services use custom or semi-custom vehicles, e.g. Deutsche Post. For those that use more "off the shelf" vehicles they are probably

Quite a few national postal services use custom or semi-custom vehicles, e.g. Deutsche Post. For those that use more "off the shelf" vehicles they are probably still custom orders specifying an exact feature set. But at the end of the day, it just becomes a matter of cost/benefit. The USPS is enormous, and that large scale makes it financially viable for them to purchase a custom vehicle since the quantities are so great. This allows them to get exactly what they want which gains some operating efficiency. Oshkosh is also a very well established manufacturer of vehicles for the government and holds substantial military contracts for custom vehicles such as the highly recognizable HEMTT. So the Oshkosh purchase is fairly low-risk in that Oshkosh has a proven track record of delivering vehicles for government fleets. UPS and FedEx also make use of largely custom vehicles. They are based on off-the-shelf step vans, but to a large extent OTS step vans no longer exist in the US and nearly all of them you see are contract designs for fleets. There's a bit of a cyclical relationship here as the modern step van is basically defined by UPS and FedEx, which purchase their trucks custom to specification from body-makers like Utilimaster and formerly Grumman-Olson. Grumman popping up here again is not coincidental - the USPS and UPS formerly got their custom trucks from the same manufacturer, but Grumman-Olson is no longer nearly as important of a player as it once was. The short answer: USPS is contracting for at least 50,000 vehicles. When you buy 50,000 of just about anything you gain a lot of leverage to customize.