You need to make contact with shipping support. There is an option within the app to make contact with shipping support. You can also use the below steps to make contact with shipping support via the OfferUp.com website.
1. Goto Offerup.com and login to your account.
2. Copy/paste this link: https://offerup.com/support/contact-us/
3. Select the first box labeled "Shipping on OfferUp"
4. Select "When will I get Paid"
5. Make sure to entter the Transaction ID and clearly state your case.