https://www.tisbest.org/redefinegifting/ Browse charities here I've found these amongst others, I think it's all American charities so I'm not very familiar with which are the good ones to support. Solve ME/CFS Initiative (Solve M.E.) IACFS/ME
As far as I can work out, all the vouchers are gone for today. I think he might release more so please highlight it here if anyone sees it. I did donate last year by getting a free gift card through this.
I think I might have missed it i.e. was too slow. It is worth signing up at the link as for example this time he sent out an email a few minutes after his tweet.