Talk to the postmaster first, even though he probably will not get anything done. get the media involved, take pictures, videos, have witnesses, dates, time, when the incident occurred and when you talked to the postmaster.
If I were you, I would call the post office and request to speak to the post master. I am sure that he would see to it that the situation was fixed to your satisfaction. I would not like my packages left on the street or in the rain either.
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