Override default shipment price through the API

We have an integrated shipping method named 'postNL'. This shipment has a default price of 6.95 (with you can set/change).
Through the API i create a checkout, finish it so it is an order. In the front-end i can override the default price of the shipment but through the API it isn't working.
If i PUT on the checkout or the order something like this
{ "shipment_method": { "id": "postnl|Default", "tax_rate": 0.05, "price_excl": 5.95, "price_incl": 6.25, "base_price_incl": 6.2475, "base_price_excl": 5.95 }
The price of the shipment is not changed. If i use for example an 'external' shippingmethod, i can change the price.
Is there any option in the API to override the shipping price on an order (or checkout with will be transformed to an order?)
Answers
I've tried this as well in the past, but never got it to work. Did you get it to work?
Nope, i still didnt get it work