This may be a silly question, but are you posting these in the same mail box? A few years back we sent a letter that never arrived, and then our neighbour had the same thing with a few letters, seems we posted in the same mailbox. We posted elsewhere after that and no more problems.  We were later told by a friend who drove by this mailbox daily, that one on occasion they saw a postal worker with a bag of mail on the ground and he was rifling through it.  These days I only post letters at the Post Office.

I've had a few go missing in Queensland recently. Although with the last one it's a tossup whether it really went missing since the "customer" was in quite a hurry to open an INR.

Queensland actually makes up the majority of my domestic sales, but I have not had one missing item from there in over 3 years.

I'm posting from WA.

Sounds like a really unfortunate coincidence 😞