Hi Paul,
Not quite sure why no one has chimed in on this sooner but yes you can quite easily adjust postage costs.
You can create a postage policies on the main hub, when adding listings you simply add the policy you want for that listing.
You then can edit the policy needed in the hub and it'll automatically change the policy for all your listings including the prices etc.
If you youtube how to make a postage policy there is plenty of guides how to do so.
All the best.