There has always been a disconnect between eBay allowing a buyer to open an INR case one day after the latest estimated delivery date, and then giving the seller three days to resolve the issue (though in reality 'resolving the issue' requires either a Tracked item to be recorded as delivered or the seller refunding the buyer), and Royal Mail's time limits for claiming for lost post (anything between 5 and 10 working days after delivery due date for UK-to-UK post, 20 days for items posted in the EU and 25 working days for International).
The above time limits are referred to in the relevant companies' Ts&Cs and are by definition accepted by anyone using those services. In the vast majority of cases items are delivered within the expected timeframe, but - as you have discovered - cause problems when items are lost or delivered late.
Unfortunately - again, as you have discovered - you are bound by the Ts&Cs of both eBay and Royal Mail, and eBay's supercedes Royal Mail's. You don't really have any other option than to refund the buyer within eBay's 3-day limit and then submit a claim with Royal Mail when their time-limit permits - although if the item is subsequently delivered between the refund on eBay and the submission of a claim with Royal Mail the Royal Mail claim would be rejected. Whether or not the buyer then 'refunds the refund' depends very much on just how honest the buyer is.
Just one more thing: in your original post you wrote "If I pay for all the lost items within 3 days, my business will die without any growth." Business? You're currently trading on a Private account...
