What kind of case did you file -- what reason for the return did you choose?
If you chose a "remorse" reason, such as "changed your mind" or "does not fit", then the seller is allowed to decline the return.
If you chose "not as described", then the seller is required to either refund you directly or provide a prepaid shipping label or pay for tracked shipping and then refund you when tracking shows delivery. If the seller does not cooperate, after three business days you can ask eBay to step in and refund you directly.
If you opened the wrong sort of case, contact eBay customer service to see if you can get that case changed to the right kind of case.
Use the Help & Contact link at the bottom of the page to request a call back from eBay. Use the "Contact us" link and follow the prompts for your issue and you should eventually wind up on a page with a chat option. Choose that, and the last option that appears in the chat window -- use the "See more" option -- should be "Contact an Agent" to request a call-back if you are in North America.
https://community.ebay.com/t5/Selling/How-do-I-contact-Customer-Support/m-p/32016431#M1783851
If using the "Contact us" link takes you directly to the chat assistant window, you may have to enter some context dependent cues to get to the call back link.
Or you can contact eBay via social media:
https://twitter.com/askebay
https://www.facebook.com/eBay/
