Thank you for your patience as I looked into this for you; the postal service is exempt from litigation - please see 28 USC 2680:
The provisions of this chapter and section 1346(b) of this title shall not apply to--
(a) Any claim based upon an act or omission of an employee of the Government, exercising due care, in the execution of a statute or regulation, whether or not such statute or regulation be valid, or based upon the exercise or performance or the failure to exercise or perform a discretionary function or duty on the part of a federal agency or an employee of the Government, whether or not the discretion involved be abused.
(b) Any claim arising out of the loss, miscarriage, or negligent transmission of letters or postal matter.
Unfortunately the automatic insurance is $50 for priority; additional insurance would need to be purchased.
It is very upsetting if a postal carrier would substitute items as that is a federal crime under the following code:
18 USC 1708- and is punishable be fines/jail:
Whoever steals, takes, or abstracts, or by fraud or deception obtains, or attempts so to obtain, from or out of any mail, post office, or station thereof, letter box, mail receptacle, or any mail route or other authorized depository for mail matter, or from a letter or mail carrier, any letter, postal card, package, bag, or mail, or abstracts or removes from any such letter, package, bag, or mail, any article or thing contained therein, or secretes, embezzles, or destroys any such letter, postal card, package, bag, or mail, or any article or thing contained therein; or
Whoever steals, takes, or abstracts, or by fraud or deception obtains any letter, postal card, package, bag, or mail, or any article or thing contained therein which has been left for collection upon or adjacent to a collection box or other authorized depository of mail matter; or
Whoever buys, receives, or conceals, or unlawfully has in his possession, any letter, postal card, package, bag, or mail, or any article or thing contained therein, which has been so stolen, taken, embezzled, or abstracted, as herein described, knowing the same to have been stolen, taken, embezzled, or abstracted—
Shall be fined under this title or imprisoned not more than five years, or both.
It would be up to the local prosecutor to pursue the matter, and there may be a proof issue (difficulty proving the actual substitution, or which employee may have been responsible).