repeat
https://www.nulledfrm.com/threads/prestashop-one-click-checkout-buy-now.10560/
demo link of module page
https://addons.prestashop.com/ru/express-checkout-process/21982-knowband-one-click-checkout.html#specifications
First. You can use for that free module mailalert
Second. U can use search on forum
try this
https://www.nulledfrm.com/threads/out-of-stock-subscription-back-in-stock-notification-module-v1-4-1.9660/#post-131923