economics 101: they are selling them to third parties for that price (including some profit and negotiation margin). So they likely pay somewhere around 20-25k for internal use. That's ridiculously cheap for this kind of purpose built vessle.
Based on this report of what the USPS is asking to replace an ages-old design mail truck [1] ($30k per), EUR 25k sounds pretty dang good for a purpose built small-run pure-electric vehicle. I'd bet they could easily get the costs down to 15K EUR with appropriate volumes.
Maintenance costs on electric vehicles are a lot less, and obviously they don't use fuel, so that costs less too.
They don't use fossil fuels, but they do need to be charged, and a fleet of them would need some added infrastructure to charge simultaneously. But that's a one-time cost, in the long run they should be cheaper to operate, clearly.
Which leaves 8K to do the customization and would not require the capital investment, on top of that they would have a much higher resale value when no longer needed.