You can calculate it before or on the pp_confirm page, or both. There really is no restriction... It soundsl like you want to show the shipping amount first... so do that. Then they go to paypal and they may change their shipping information, so you should probably do it on the pp_confirm page as well. Really anything is possible. It is up to you to decide what to do and then if you run into problems you can debug them and get it working exactly how you want it.