Sorry you are going through this, OP. We had similar.
We did the cage on the letterbox thing to catch letters and intercept them before the kids got them, that solved that one problem.
The parcels were different again. They were always sent signed for and we had a run of them that were signed for by neighbours who then dropped them round to us, which with hindsight probably drove my mother mad. I did not respond to these just charity shopped the contents.
I think on one occasion I was out when a parcel arrived and there was a summons from the post office to collect it within a particular timeframe, which I ignored.
After three years of this, plus emails, letters, emails from email addresses they had created as they realised I had blocked them we sent a cease and desist letter stating that we considered their actions to be harassment.
They responded with not one, but two solicitor's letters.
I reported them to the police for harassment. They were given a warning and that finally stopped it.
I'd also just like to say that you will probably get some well-meaning replies on this thread about how you should "just" do this or that. Those posts might be well meaning but they have no understanding of just how traumatic it is to finally lay down a boundary and then be stalked in your own home - and someone toxic who you have finally had the strength to cut out getting stuff into your own home is extremely traumatic.
I had a shit therapist at the time, btw, who just used to suggest I "return to sender" and also said that I "failed" to collect the parcel from the post office. I didn't "fail" to do anything, I just lived my life, I don't dance to the command of my toxic mother.