Here is that the American Philatelic Society says regarding their expertizing service:
Quote:
All reasonable care will be taken of items submitted for examination, but all items are accepted with the full understanding on the part of the owner that neither the American Philatelic Society, the American Stamp Dealers' Association, nor members of their expertizing committees are liable for any loss or damage resulting from any cause whatever except for gross negligence.
So you would be responsible for arranging for any insurance that you feel you would need. I think that standard stamp collection insurance policies in the US would cover this.
To protect the stamps for only transit to/from the expertizer, you could perhaps purchase insurance from the shipping company.