There is an issue with many users right now that aren't seeing the shipping option.
I believe OfferUp is trying to determine the cause of this but is still unknown. Hopefully this will be resolved with the next release.
There are a few things you can try.
-Re-validate your email address - even if it's already been done!
1 . Log into the OfferUp app or website
2 Select your Account icon Account page
3 . Tap edit to the right of your email address
4. Enter your email and Select the Confirm Email option . I've had to put a space after my email to get the 'confirm email' option.
Then, close the app and reopen.
Here's more on troubleshooting this issue: http://ask.offerup.com/Shipping/1070721721/I-don-t-see-shipping-in-the-app.htm
Also, the option will not display if the item isn't eligible for shipping.
I hope this gives you some options to try to get it working. One other mention...If you have another phone you can try that as well! I got mine working again when I went to an android phone (I use iPhone).
Best of luck!