Proper procedure for getting a return through eBay AND keeping the protections of the Money Back Guarantee (MBG) is to file a "case" in the Resolution Center, link at bottom of the page. It is not clear from your post if that is indeed what you did? You have 30-days from when tracking shows "delivered" to avail yourself of the the MBG for returns.
If the seller is required to, or has 'free returns,' the the seller will send you a return shipping label through the eBay system. Anyone who tells you they are mailing via USPS, snail mail, is not following procedure and possibly scamming. You should get a label and then be able to print it.
There is much more detail, so if you want to wander into the weeds, take a look at the MBG:
https://www.ebay.com/help/policies/ebay-money-back-guarantee-policy/ebay-money-back-guarantee-policy...
