I have been doing this for well over 20 years, on & off. Just lately it's required to "un-learn" a great deal of  experience. The changes are not subtly introduced with adequate and clear guidance, but arbitrarily imposed. Sellers now need to learn how to circumvent the simple delivery system as it sometimes fails in certain situations. I would suggest not offering free postage to buyers until you can familiarise yourself with SD and ways of avoiding it when necessary.