how do you collect your refund if it was insured and you are asked what was in the package.
You file the claim based on the value of the package, not what was in it. And even if they do ask about the contents it wasn't illegal in the first place so there's nothing they can do about it, the stupidity of two clerks notwithstanding.
Frankly, if I were you'd I'd be filing a formal complaint against the two clerks with the Post Office Inspector General. If they ever do that again, demand a complaint form - they have to give you one and provide a receipt for it when you turn it in. And be sure to get their names along with the form.