Some users have experienced the shipping option disappearing/reappearing.
If you have any items up for sale that do not qualify for shipping (weight over 20lbs...like furniture, appliances,etc and/or items valued over $500), try archiving them and recheck.
Have you tried following the steps listed here?
Make sure you're using the newest version of the app. More about updates
Try quitting your app completely and relaunching it to be sure you're seeing the latest info.
To be able to post shippable items for sale, validate your email address with us, if it hasn't already been validated. Manage your email settings
When you figure it out, please do come back and share as to what the issue was or how it was resolved, as it will help others in the future. Thank you in advance for your cooperation.