No, it shouldn't have been sent at such a high voltage. You are right, storing and sending a battery fully charged (which it must have been to arrive at 8.3V) will cause it to puff.

The way he tried to explain it sounds very condescending and is also just incorrect. Clearly he treats all his batteries this way.

I'd suggest requesting a partial refund or ask to return the car minus lipo for a full or mostly full refund and be done with it. Or if you're happy enough with the car, just don't deal with this guy again and post to a RC Good/Bad List on facebook to help others avoid this seller.