I would do nothing for the moment.

There is no guarantee that AP won't charge for the return. I would wait (probably 6 weeks) for the parcel to get back to you before I did anything.

If the buyer opens a claim, you should win, as failure to collect is not a legitimate reason for an INR case.

When you get the parcel back, you can then determine what or if you will refund. I would be only refunding less all costs - postage, packing, eBay and Paypal fees. Paypal no longer refund fees and eBay won't for a partial refund, and after that amount of time probably not even for a full refund.