J.T.
My SIL has issues with their delivery person and they have called the post office several times about it - esp. when they were expecting things in the mail that they did not get. The one time, the post master found a whole stack of their mail in the back room - 'no one' knew why it was there.
I don't know if it would do any good for you to call, but it sounds like maybe she needs to call. The other thing that you *could* do is to send it registered mail, or priority, or something that would give you a tracking number. Seems kind of silly for just mail, but at least you would have proof that it was sent....