First, report the possible identity theft. Please see the link below.
Suing the federal government, which the USPS is a branch of, is difficult, and you have no damages at this point anyway, other than the rude treatment, which you can report to a USPS supervisor.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.