I don't know exactly what you're looking for, but you might want to check this out.
This is the website for PostNL API's: https://developer.postnl.nl/api-overview/
For delivery date there is a specific API: https://developer.postnl.nl/api-overview/delivery-options/deliverydate-webservice/
And PostNL has a fixed pricing. You can learn about it here: https://www.postnl.nl/zakelijke-tarieven/2024/
They change on a yearly basis and depend on the weight and size of the package, but that makes in total four different prices.