A buyer cannot open an eBay Dispute until the last day of the estimated date for delivery. Then she has 30 days to open an Item Not Received (INR) dispute.
If she is opening Paypal disputes, you have to promptly give the tracking number, and it doesn't hurt to give the shipping date and service used as well.
Are you responding with the tracking number.?
That is your defence.
I understand there is an appeal when a refund has been made. But the refund comes first.
And drop the No Returns policy.
You are allowed to have one, but it is one of the reasons customers open INR and NAD disputes, figuring that the seller is not going to cooperate if there is a real problem.
You can have a No Returns policy, but you cannot have a No Refunds policy.