Good posts.
I personally, if I could, would get her some presents from a charity shop. Some that perhaps dont look like they are definitely from a charity shop.
Christmas [or the show of lots of presents], seems to be very importnant to her for some reason.
I dont think it is worth upsetting her about it.
I know that it is a bit like caving in, but that is what I would do if I could.
She will probably put your presents at the back of the tree or something, but she should realise that you did your best.
She is doing wrong behaviour about this, so you shouldnt feel at all bad about it.