The mail carrier uses personal vehicles here too but I think they may be contract workers not federal employees? But still they are in charge of federal mail so there should be something against them smoking around the mail being delivered. I would be waiting for the carrier if my packages arrived smelling like that with a bill either for cleaning or Dr. visit. And back it up with a copy of the letters I would be sending to anyone connected to the PO and to my lawyer. Then I would get a PO box because a person that has no respect to smoke around other's belongings would not be passed losing a piece of important mail that you need.