One of the exceptions is low priced items. What that means I have no idea but that is what I will be listing. You will not be able to use your own stamps, otherwise you would be paying twice, stamp and ebay label.
Simple postage is only suitable for those who cannot work out their own postage, so ebay guesses for them, and costs many with the mistakes. Also ensures use of dodgy couriers who ebay should never suggest using.