I now have to wait in and as I WFH I might as well work until the delivery arrives.
I think I have spotted the misapprehension you are labouring under
You really don't have to stay in for the delivery.
Just go out, let it fail.
If you are not comfortable with that, you could just tell your "friend" that you are sorry, it won't work for you to accept this delivery.
And in future, avoid her , if she hasn't heard back from you that you will do things, you can not do them with a clear conscience.
Every time I give an excuse for saying no, she then comes up with a million ways to make it work so I can do the favour.
So don't give an excuse. Smile, say sorry and say nothing else
Please consider the possibility that your finance is frustrated by your inability to prioritise yourself over some random p**s taker. I would struggle with that in a friend, never mind in a fiancé.
Why on earth have you decided that you have to wait in for her convenience when she knows that it is not convenient for you
I repeat my advice to not accept the delivery. You see when you ask her for a favour she just ignores the request and doesn't do it. Be like that with her. Please
Overall, though, follow the excellent advice above re building your own self esteem @catwomando