Do you mean as the seller? Put all the items into a box with the packaging and weigh them. Then hop onto the AP website and put in the details, that will give you an accurate price to your buyers destination provided you have entered all the details correctly.
If you mean how to combine items on an ebay invoice, add the postage charge, DO NOT click recalculate, preview and send.
If it is something else you will have to explain further, as my crystal ball is out of batteries today 🙂
