Yep, you are right the buyer cannot claim anything through eBay as they have a 30 day MBG policy, but they can turn to their payment provider.
Most card companies offer a longer period of buyer protection and if they paid with PayPal they have a 180 day buyer protection.
You must always keep those proofs of postage safe for at least 6 months, I keep mine for a year.
Without that proof of delivery you will have to refund in full.