I did not read all the replies so if this has been said please forgiv.
Yes I would view this as a threat, as well as liable(since it is in writing).
Forget the elders, go to the police and be sure and tell them that you are afraid for youself and your children. Tell them you want it served now.
Next go to your postmaster with the envelopes and let him know that this person is using the mail box illeagly. It is a crime to put things in someones mail box that has no mark. Tell the postmaster that you are afraid that she might put a bomb or some poison in there. Since she is spying on you she probably knows you pick up the mail.
I hope things get better for you.