It will probably depend a little on what listing format you have (there's the old listing form, a new listing form, and a 'quick' listing form with minimal options), and/or whether you've been opted into business policies (good ol' eBay - keepin' it simple ).

I should be able to advise better if I can confirm which type you have, (if you're not sure, a quick screenshot that shows the postage section of your listing form should do).