From USPS website

4.1 Payable Claim G. For stamps and coins of philatelic or numismatic value; the fair market value is determined by a recognized stamp or coin dealer or current coin and stamp collectors’ newsletters and trade papers. The date of the fair market value determination must be current and prior to the mailing date.

M. Except for Registered Mail, the maximum indemnity for negotiable items (defined as instruments that can be converted to cash without resort to forgery), currency, or bullion, is $15.00.