Was the item tracked?
Does the tracking show delivery?
Does the tracking show the item still in transit?
What was the last estimated date for delivery?
If you did not use tracking, refund. You cannot win a Not Delivered dispute unless you can prove delivery.
If you force eBay to refund the customer you will get a Defect* and eBay will take the money from you anyway.
If you did use tracking put the number in the Dispute.
You will get a short respite to allow for delivery. If the tracking shows delivery you win the dispute and the buyer cannot leave feedback.
In my 30+ years of mail order, it's been my experience that overseas shipments (Air Mail) take 20-30 days for delivery. Was the date March 1 or March 21?