How do I stop the letters?

I’ve been NC for about 3.5 years since my mother’s boundary stomping rapidly increased and she had a meltdown when I asked for space (as per my previous posts). I’ve had a lot going on since then (multiple rounds of ivf, which she doesn’t know about, plus the obvious lifestyle changes brought about by the pandemic). I live on the other side of the world to her, and she’s blocked on phone and email, but right from the start of NC she decided that she was going to send me a continuous stream of letters by post (I had spoken to her in the past about that and had asked her not to send me letters, they’ve always felt so intrusive, although I didn’t say that). I hadn’t initially intended to be NC for so long, but when the first letter arrived and I read it (it was full of denial and emotional blackmail, delusion and gaslighting), I realised I needed to be NC longer, to take more space for myself. After that she started sending letters by post every couple of months. I never opened them but have kept them all unopened in a shoe box. There were also some flying monkey events. Every time I received a letter it felt like a big ‘fuck you and your boundaries’ from her, and generally upset me for days (she always seemed to send them at perfect moments, such as the day of ivf treatment, even though she couldn’t possibly know about that as I’ve not told anyone who’s in contact with her). Anyways, about 18 months ago my brother queried when I might speak to her again and I told him I’d probably need to not receive any letters (or any harassment from other people on her behalf) for at least 6 months, then I might consider speaking with her if it felt right. I had hoped he’d told her that. After that the next few letters were about 4 months apart, and it appears to have been 6 months since the last one, so I had finally had some hope that she was respecting my need for a break! Until today that is, when I received another one. I won’t open it of course but I’m so upset and disappointed. I have her blocked on all other means of communication, she knows I asked for space and that I don’t want to receive the letters, and I’m sure she must realise that every letter she sends is lengthening the NC, so why does she do it? It really does feel like a ‘fuck you’. But do you think there’s a chance that she doesn’t understand? i.e. that she really thinks that sending letters might somehow make me want to be close to her and see things from her point of view (despite a lifetime of abuse, lying, manipulation, drama, rage and confusion that she’s in complete denial about)? I wondered whether I should put all of the unopened letters in an envelope and send them to her, as it might make her realise that I haven’t been reading them. Or whether I should ask a cousin to tell her to stop sending them to me. But both of those options might create drama (which I don’t want, and which would, in a way reward her for her behaviour as she thrives on drama). I just want her to realise that she needs to prove she can respect a boundary or a simple request for a bit of space (I only asked for a couple of months initially) before I’ll ever trust her again.