You can respond to the dispute, but if escalated to a claim it will almost certainly close in your favor straight away - PayPal do not entertain claims for partial refunds and/or for differences in postage costs, and even if by some strange chance the claim for "not as described" is successful, the only thing that will happen is that the buyer will have to send the item back by trackable post to get a full refund.

Your buyer's claim is, IMHO, ridiculous.