During the listing process if the Custom Postage option is there select that to change postage. If not try opting out of Simple Delivery. During listing click on the postage options icon on the right of the postage section and remove details of parcel weight and dimensions. Then you should be able to select what postage you want.
Also if you buy postage outside of ebay remember to mark dispatched and add the tracking number (if there is one) manually via the dropdown menu arrow for the item on your orders page.