There will always be complain kings who find fault with everything and open INR cases simply because they’re impatient. But if the EDD is not over yet, all you have to do is provide the buyer with the tracking number again and reassure them that the item is on its way.
In your message, you should also remind them of the stated EDD and emphasise that the carrier needs enough time to make the delivery. Let your buyers know that you are there to help, and ask them to reach out to you first to resolve any problems. You don’t want them to escalate an INR case to eBay straightaway!
Additionally, check with the carrier to ensure that the delivery can be made by the EDD so you don’t end up giving empty promises to the buyers.