We note that INR case was closed in your favour.
That means as far as eBay policy/ T&C is concerned matter is closed and you are not required to issue any refund to buyer. It is very good of you that you are prepared to offer partial refund.
If Payment was through eBay's payments then simply send buyer a message detailing your exact costs which will be original postage, any return to sender postage charges, eBay's fees ( These are not refunded as you are not issuing a full refund to buyer) and advise your buyer that you are going to issue a refund for the amount your buyer paid minus these charges. Buyer does not have to agree or accept your proposal as he is not covered by eBay's money back guarantee and INR case is already closed in your favour.
If payment was through Pay pal and your buyer opens an item not received case through Pay pal then Pay pal will require you to provide documented evidence that Item was DELIVERED to address as shown on record and sadly you can not provide this evidence as item was returned to you because address was incomplete.
Contact Pay pal who no doubt will sympathise with you but Financial services authority who make the law regarding payments demand that Pay Pal simply deal with delivered" Yes or No" and do not consider anything else including failed deliveries due to incomplete address, However pay pal at their own discretion may refund the buyer and not deduct the amount from your account.
We know it is difficult to accept that buyer is expecting you to be out of pocket for their error but for sake of sanity do not take it personally and ignore his comments which are a sign of his frustration at his own error and he thinks he can get what he wants by bullying which is a common trait these days. It takes a big person to admit and pay for their error.
If Pay pal is not involved then there is nothing he can do and you should add him to your Blocked buyer list.
Also report the buyer as this will help you should the buyer leave negative feedback and eBay should remove this but you will need to contact them.
Hope this helps and you surely did not need this extra hassle in these difficult times.
Good luck
