Have you tried contacting them again and making that point? Or maybe the return shipping isn't all that much on a small package like a iPhone case and it's not worth the headache or risking your positive feedback rating. (Or did I read something a long time ago about how sellers aren't allowed to give negative feedback any more?)
Coincidentally, I'm in a similar boat right now. I've had very good luck with eBay over the years, but today I got an e-mail from UPS saying that an item with a specific ETA date won't be here until after I need it. I wrote to the seller and they were kind of like "too bad, sorry--estimates are just estimates." (Agreed, but he had the ability to send it in time but didn't.) I'm annoyed but I suppose he's technically right, and stuff like this is the price of doing business (and saving money) on eBay.
I'd suggest contacting the seller one more time, pointing out that the mistake wasn't yours (sometimes I've softened my tone by suggesting that it was an honest mistake on their part, and that maybe their supplier was providing them with the wrong, sub-par merchandise). If you wanted to play hardball you could even say that you don't want to open a dispute about such a small matter. That would surely get their attention; it may not be fair, but buyers do have a disproportionate amount of control over the feedback process.
I had a similar issue with a stinky, non-leather Kindle cover, and the seller was awesome about taking it back and paying the postage, but apparently the seller's opportunity for awesomeness has already been lost. Good luck!
