Keep hounding them. I've had terrible experiences with their customer service but I have managed to wear them down by being persistent. I've purchased from them twice, both times had issues with my order, and both times their first response was the equivalent of "too bad".

Also, contact Schwalbe and ask about your issue. If they are willing to back you up, then either they will probably take care of you directly or at least you can use that to pressure PBK.