I just had a bit more a look around. If I access the postage policies through my account, I can add a handling fee. It doesn't allow me to delete my policies as they are in use or a default.  I am not sure how you get your option for domestic postage, but at least I managed to locate the adding of additional cost. I can specify it on the policy if I edit it through my account/policies instead of while listing the item. This means I may end up with multiple policies just to cater for different packaging. Convoluted but workable 🙂

Thank you for all your help. I really appreciate you taking the time out to assist with so much detail.  Thank you, thank you, thank you.