Then you can go to the developer console in Chrome and remove the disabled flag on the 'Out of stock' console button. With the button now enabled then you can add it to the basket and checkout.
Worked for me, got an order through 10mins ago, apparently to arrive on the 12th. Fingers crossed.
One for the slightly geeky
If you go to
https://www.microsoft.com/en-gb/store/configure/Xbox-Series-X/8WJ714N3RBTL
Then you can go to the developer console in Chrome and remove the disabled flag on the 'Out of stock' console button. With the button now enabled then you can add it to the basket and checkout.
Worked for me, got an order through 10mins ago, apparently to arrive on the 12th. Fingers crossed.