I presume you are the seller (if you are a buyer, you file a case with ebay as item not as described).
As a seller, you will need a cooperation from the buyer. Ask them to keep the packaging material and take photos.
You file the insurance claim with the entity that insured the package. For example if you mailed it via Priority (insured automatically up to $100) you file with USPS. If you insured the package with a third party, you may need to file with both, the USPS and the third party. For example. You mailed you package via Priority, declared value $150 (i.e. over $100) and insured it with a third party (you can do that when you use stamps.com or pirateship.com or other label providers). In this case you file with the third party but they will only cover $50 and will ask you to get the rest via USPS.
If you sent it 1st class and insured only with a third party (for $150), you submit the claim with them and they cover the entire value.
You don't file (as a seller) with ebay. They have a new international mail program, where you can insure your package via third party, but I am guessing your question involves domestic mail.
