I'm not sure about earlier Doblos, but my 2017 model has independent rear suspension, which is why it was popular with the wheelchair adaptors, allowing an easy floor lowering.
Whether yours is a van, or a car, it will be heavier than a Punto, so needs stronger springs. Any lowering springs for Punto will be for the car, not the greater load of the Doblo. You'd create a vehicle that you daren't carry your shopping in. There's also the legal aspect. Lowering springs will change its load carrying capacity, so its original type approval and rating swoudl then be wrong. To do it correctly, you'd need to go through the 'Single Vehicle Approval' route, like kit cars. If involved in any collision, even a parking knock, your insurance company would probably abandon you. If you inform them of the mod, and you should, they'll ask difficult questions, and may need proof that the springs were designed for your Doblo.
If you really need it to ride lower, carry a couple of bags of sand.