Im looking for a way to change the shipping price if all products in a cart are from a certain category.
Looked into promotions Where I can make a rule to match all products from category X but cannot change the shipping price, only set it to free shipping.
You will need to roll your own shipping module.
This tutorial has what you need to know to get started:
http://www.excellencemagentoblog.com/magento-create-custom-shipping-method