Your claim if any, against the USPS would be solely for the postage and perhaps the contents of the package if you had insured it prior to shipping.
Your action against the person who opened your mail, likely is limited or minimal - what were the specific economic damages? Embarrassment is not worth a whole lot. You could report it to the USPS and see if they want to take action against the other individual for the 'wrongful' opening of another person's mail, but do not hold your breath waiting for satisfaction via this path. They might fine him up to $50.
Title 18-Part 1-Chapter 83-1703 states in part that "Whoever, without the authority opens or destroys any mail or newspaper package not directed to him shall be fined under this title or imprisoned not more than one year or both."
Your best option is probably to set it aside, put it behind you, handle it differently next time and move on to other more pressing matters.
Disclaimer Information on this site is provided by Brian Scott Wayson as general information, not legal advice, and use of this information does not establish an attorney-client relationship. If you have questions about your specific situation, please call an attorney.