If they can custom design a vehicle that saves a driver 1 second per delivery, or decrease their accident rate by 1%, that could be enough savings to pay for the vehicle and then some.
Many organizations that operate at a huge scale find it beneficial to customize their essential tool set.
That being said -- The USPS definitely does have some vehicles from traditional automakers in their fleet. My mailman drives a RAM ProMaster. https://en.wikipedia.org/wiki/Fiat_Ducato It's left-hand-drive though, so I would imagine that carriers wouldn't want to drive one on a route with mailboxes at the roadside. https://hips.hearstapps.com/hmg-prod.s3.amazonaws.com/images...