Hiya nanny' the only lowlife maggot atm is your neighbour, I'm afraid, and tho I expect you'd like to post her back through her own letterbox head first I really do feel that it's time to involve the polis to sort her out, once and for all.
I know that you drew a blank first time but upthread someone gave you a link to a specific law, so I reckon nows the time to tell them of this if whoever you speak to is as clueless as the first bloke.
I think this because she seems to equate the parking stuff with this and operating on a tit for tat payback , but really, it's not like that at all.
When she was asked to take in the parcel, she had the option to refuse,and what would have happened then? Well, either they'd have tried another neighbourv or they'd have returned it to sender?
But she agreed to take it, has actually seen you since, has had ample opportunity to pass it to you.
So I really think that you can't give her any leeway. She's got to learn that her actions have consequences.
Silly moo.
All the best.