Having some insight to the workings of the USPS, I can share some insight.
For one, the postal inspector is a Federal Agent, and a local cop is not going to leave a card behind acting as one. It would be no different that one acting like an FBI agent.
Normally, pending the size of your community, the PI isn't at the local post office, unless your post office happens to be the metro headquarters.
Normally, but not in all cases, the PI's office will not engage in drug investigations, unless the use of the USPS was used to deliver those items. If the USPS was used, the USPS PI office has the full power of the Fed Gov and will work with the FBI to complete the investigation. They are able to obtain warrants and search and seize like any investigative office.
As my ex-wife can attest...they do not fool around, (she stole my mail years ago with my bank statements). Break the laws using the USPS and you could find yourself in deep shit. You're not dealing with Barney Fife here.
If you have done something illegal, and they want you, they will find you. Since they left a notice, chances are they just have questions that could be directed towards possible mail fraud etc. It could be nothing, but then it could be something. It could be about a neighbor whose mail was stolen and could be asking if you noticed anyone around their mailbox that was not them.
I would follow the advice listed above, clean out your place, call the PI, and don't answer any questions pertaining to illegal activity that you might have participated in, and if they push, say that you can't answer any questions regarding anything without legal council. Cops get away with this one all the time...just have a few questions to ask and the perp gets busted because he fell into their relaxed state of questioning.
As stated, I would call once you have place in order. If not, they might believe you do have something to hide.