http://new.usps.com/cpim/ftp/pubs/pub122.pdf

As it states here.....

WHO CAN FILE?

The sender or adressee may file a claim for damage or loss of contents of registered, COD, insured or Express Mail article. When the claim is for complete loss of a registered, COD, insured or Express Mail article only the sender may file the claim.
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STRANGE STATEMENT.... But the last sentence says it all. The first sentence is kind of misleading. As "LOSS" can be complete can't it?

Then again maybe I am reading more into this than I should. Maybe if the envelope was received without the item enclosed, then it isn't considered a complete loss. The more I read it the more I believe this to be the case.

Yet, in the case of an item missing from the an envelope, it would be better if the Sender and the recepient both take some action with the post office at the same time as the recepient wouldn't have the original form and could be limited in the amount of reimbursement.

It appears that an insurance claim for a "complete loss" is the main burden of the "THE PERSON MAILING THE ITEM".

On another thought, I'd hate to have the "seller" send me the original receipt of purchase of insurance that the post office gave them. They might not be smart enough NOT to send it in the package along with the original purchased item. Then again sending it in a second envelope might not be smart either, as it's just another item that might get lost. (Fairly uncommon with the P.O. but it can happen)