YMMV if you don't eat the shipping cost and issue a full refund. eBay's written FVF refund/credit policy only covers sellers voluntarily issuing full refunds. Either way, buyer needs to issue a return/cancel request, you need to do your thing, then you need to contact customer support. You might have to escalate to their resolution center and a supervisor.
You're also probably going to be out the PayPal transaction fee.
Your alternative to a partial refund is to issue a full refund then contact eBay or PayPal's resolution center to get reimbursed for the buyer's shipping error.