Hi Joha,
With today's email ("I'd like to get these for the kids. I think they are very _____. Please let me know if this is okay with you.")
Oh how that sounds familiar... .Yes, it is crazy how such a 'simple' request can result in such high stress levels.
For my mother, I think the main issue is placing the responsibility with me. A gift that is not met with the correct amount of enthousiasm feels as a personal failure, and thus as a rejection (her worst fear). But if I am the one suggesting or approving of a gift beforehand, she can put the blame on me, and avoid feeling rejected. I haven't yet found a way to avoid the barrage of mails, but at least I can now see why she is stressing about this so much.
I'm considering telling my mom a few things. I'd like to be honest, but not hurtful, but blunt. These things are:
1. generally about my stress level and mental health state
2. in relation to that, how I feel about the holidays and gifting (which she's already aware)
3. she doesn't need to run every gift past me. I don't have time to coordinate nor do I enjoy it
4. use your best judgement regarding the gifting, and think of how it'll go over (I've given her a few basic guidelines before)
Juggling a full-time job and children is not easy. Doing this almost solo requires all your attention and energy, I am sure!
I have been struggling with my health and stress levels as well. My mother knows I have been ill, but not to what extent. Telling her that would only make her fret, and would uppen her attempts to boundary-bust with 'well-intended help'

In my case, I am trying to communicate points 3 and 4 to her. Not in one clear mail or talk, because that simply doesn't work: she shuts down when trying to talk things through and simply stops 'hearing' me. Instead, I am trying to throw every ball back into her camp, getting her to take the initiative again. It's an intricate game of ping-pong, but I hope this way she will get the message in the end. It has worked for me in a couple of situations these last months, but of course every relationship is different, and it might not work in your case.
I hope you find some way to get the spamming and the related stress under control.
Libra.