Empty box dispute

Any advice ? I received my first empty box dispute. Sent 3 pairs of shoes and the buyer claims they received 2 pairs and 1 empty box. Certianly was not sent that way. I do have a postage receipt that confirms a 1 to 2kg package. 2 of these items and an empty box would be less than 1kg. Also this dispute was started 17 days after the package was delivered. But ebay say they have 30 days. That seems odd in the case of a missing item empty box case

I have reported to ebay but they say they have to speak to both parties. Meantime they have also left feedback saying they received an empty box which annoys me further