you didn't follow seller protection rules(which used to mean signed receipt
of delivery; proof of postage was worth zilch)
Did you send it signed-for?
No longer true. For the last year ish. As long as the item sent was parcel sized (not letter or large letter), a proof of postage will have a reference number on it, this can be used to get a delivery status from the RM website.
This is enough to win a not received dispute with eBay or PayPal (as long as it shows as delivered, obviously).
The only issues with this service is that you’re only covered for up to £20 compensation should the item get lost. If it’s over £20 but under £50 then you may want to consider paying the extra for Signed for.
To save any issues you’re always best uploading this or a Signed for reference number against the sales record as soon as the item has been sent.
Back to the OP........ if the buyer opens a not received case with eBay, you won’t need to refund, no. Any cases opened with them after 30 days of the end of delivery estimate, are for communication purposes only (ie eBay won’t force a refund).
However, if they open a PayPal case then their rights extend for 180 days, so you will have to refund yes.