Recalculate Shipping on Backend Order Changes?

Hello,
i created a Shipping Extension, that calculates the shipping costs dependant on the product categories.
Example:
Category A -> 35€
Category B -> 10€

It works fine in the frontend. But when I delete or add new products in the backend, how do I tell OpenCart to recalculate the costs for shipping?
Is this behaviour supported by ANY shipping method?
thanks
ckonig

Seriously, this cannot only be a problem for me? Did nobody ever ask this?

I'm not quite sure what you're asking...why do you need to recalculate shipping costs when in the back-end? How exactly does your extension calculate cost?